To Atra2 and Sejs: My point wasn't that Fighter is weak or useless for multiclassing. The bonus feats are great for multiclassing, but mostly just for taking a few levels of Fighter and no more. And Complete Warrior has no less broken stuff than Sword & Fist did, though I guess it isn't as bad as Masters Of The Wild. The Fighter can't really stand on its own, it desperately needs to multiclass in something else to be effective, and even then a Fighter is never as effective as a Barbarian, Ranger, Paladin, or any mid-to-high-level spellcaster. The Fighter needs many buffs from a Cleric and Wizard in order to hold up well in battle at middle and high levels, and can't make use of the many Cleric and Wizard spells that are self-only and completely stop various effects or attacks (Stoneskin, Major Sphere Of Invulnerability, Prismatic Sphere, etc.). The Fighter is useless beyond 4th-level or so without a good magic weapon and magic armor (well, not quite as much a problem in 3.5, but then, a 3.5 Fighter needs adamantine and mithral and cold iron and alchemical silver weapons as back-ups as well or they're just as useless, which is expensive and absurd).
The Fighter is entirely dependant on Kewl Lewt and those Magic-Using Egomaniacs He Must Always Suck Up To In Order To Survive (tm). The lack of those, or just very little of them, results in a useless Fighter. And Fighters just don't have the versatility or raw power to make them worthwhile like that. Only horribly broken supplements, feats, prestige classes, and such can make playing a Fighter, probably not even a pure Fighter, a good and effective choice; and that defeats the point. Which sucks, cuz as I said, Fighters should be scary-powerful in their battle skills at high-level, like Sephiroth or Conan.
I figure the classes are in roughly this order of usefulness, from most to least: Cleric, Wizard, Druid, Rogue, Sorcerer, Paladin, Barbarian, Bard, Ranger, Monk, Fighter.
Bards are effective support characters and can make an entire party fight more efficiently and effectively. They can heal, they can buff, they can charm, they can produce illusions, they can even blast stuff just a bit. They have enough skills and skill points to effectively fill several roles, even if not terribly well sometimes, and they can generally accompany anyone else in the party for almost any task (need to scout ahead? no problem, I'll go with you, and I can even make us invisible; need to ambush that monster? I'll sneak around the other side and flank 'em; need to get on the Baron's good side? No problem....). Bards have enough combat ability to help flank in combat and to help protect the party's major spellcasters, and they can be decent in battle once buffed up with their spells and bardic music. They're very versatile and DO NOT suck at everything they do, there's just not many things that they specialize in; what they do specialize in though is stealth, misdirection, deception, socializing, controlling the situation, disguise, support, and knowledge. Yes, Bards are a generally weak class, with only mediocre combat ability and spellcasting and skill selection, but largely make up for it in the versatility of all this together.
Barbarians have more raw offensive and defensive power than Fighters, and more general versatility, though not as much as Rangers or Bards.
Clerics can heal allies, enhance allies, blast enemies, cripple enemies, divine the future, divine clues and lore, summon allies to fight for them or perform other tasks, fight well in melee or ranged combat especially if using their spells to pump themselves up, survive attacks that would kill other spellcasters, and control or ward off undead. They are very versatile and solid, very capable and especially so in their specialties. Only an inexperienced, foolish, or old-school-jaded player would play a 3/3.5 Cleric as just a boring healer.
Druids can do several things that Clerics do, though sometimes not quite as well, but they have the advantage of greater destructive power at some levels and better versatility outside of battle. Their combat ability is a bit worse due to their armor/weapon restrictions, but their spells and wild shape ability can make up for that. They're very potent in the wilderness, of course, though about half of their spells and abilities are useless elsewhere (still leaving them with mediocre combat ability, wild shaping, animal companions, and such though).
Monks are more versatile than Fighters and Barbarians besides combat, and have combat versatility somewhere in between. They could definately use more skill points though, and they need too many high ability scores to be effective and survivable. They do have many cool powers at middle and high levels, but even then they have difficulty surviving due to their mediocre hit points, reliance on staying in melee, and low to mediocre attack/damage rolls. They need d12 hit dice or better attack bonuses to be effective, but then they might need to have a few of their special powers removed.
Paladins are very effective against undead and evil creatures, but lose some of their offensive and defensive abilities when fighting other types of foes. They have only a little more non-combat versatility than Fighters do, and greater survivability. They give up some of the combat versatility of a Fighter to instead have greater power against a few types of enemies, and for the sake of gaining a special mount and healing/buffing spells. They are not useless against non-undead, non-evil enemies, though they are only about as effective as a Fighter in those cases, with the advantage of healing and buffing even then.
Rangers have greater non-combat versatility than the other warrior classes, though half of that versatility only applies in the wilderness, which still leaves Rangers at least more sneaky and observant regardless of environment. They aren't as versatile in combat as Fighters, but are more effective against a few types of enemies, and have enough usefulness and effectiveness outside of combat so as to make them better than Fighters overall. They're more accurate and deadly in combat than Monks, so they can survive somewhat better as they can actually hurt enemeis and slay them without wasting as much time missing with their attacks.
Rogues are the only good trap-finders and trap-disarmers amongst the core classes, which at least leaves them an important niche, and they are almost as good as Bards when it comes to sneaking and deception (lacking only the Bard's spells of illusion, charm, and stealth). Rogues have great versatility with their skills, and have enough combat ability to be useful when their skills don't apply. They can do great damage with sneak attacks, and can use Bluff, Hide, and Move Silently to improve their chances of getting off many sneak attacks each battle. Their combat ability is cut in half against creatures they can't sneak attack, but they at least retain decent defensive abilities, a decent attack bonus, and survival abilities like Evasion, Slippery Mind, Uncanny Dodge, and Defensive Roll. They could certainly use a bit more sneaking ability and defense, but are reasonably effective as-is. A skilled rogue doesn't get hurt or killed by traps or scouting very often, unless they're foolhardy or horribly inexperienced (generally just player faults, really). Well, unless their DM is evil, anyway, in which case the rogue is about as likely as anyone else to die, they just have better chances of dying seperate from the rest of the party who's about to be TPK'd by an RBDM.
Sorcerers are fairly powerful, they just lack versatility and some defense. They only learn a few spells of each level, so they can't learn every defensive or offensive spell for each possible type of attack against them, but that's why they must choose their spells carefully and take stuff like Fly and Invisibility to stay out of harm's way (or out of its notice) for as long as needed. They can cast their spells many times per day, spontaneously, which is very effective, simply limited by the small number of spells they know. A sorcerer rarely has to worry about their magic being dispelled since they easily have enough spell slots to re-cast it again. Having multitudinous Magic Missiles, Mage Armors, Lightning Bolts, Hastes, Flies, or Tenser's Transformations each day can be pretty powerful. Sorcerers only lack in versatility and uniqueness (they steal the psionicist's original shtick and flavor, and squander it). They need a few more spells known, more skill points, and a superior class skill selection. A better hit die or base attack bonus would make sense with their flavor text, but would probably be unbalanced unless their spellcasting was reduced (say, slowing their spell progression a bit and limiting them to spells no higher than 7th by 20th-level).
Wizards are just plain powerful. I think that's about all there is to say on that. If I need to explain it, there's something wierd about how your group plays, or you're a total newbie. Not trying to flame, just sayin', most groups quickly and clearly witness the raw power and versatility of a Wizard in due time. They could use a few more skill points maybe, or better benefits from specializing perhaps, but it'd probably just be overkill.