There are two big type of gestalts: synergy gestalts and complementary gestalts.
An example of complementary gestalt would be sorcerer/barbarian. Good spellcasting and Will save on one hand, good BAB, HD, and Fort save on the other, plus various nifty abilities. It's a solid, all-around competent character.
Synergy gestalt use two similar classes which are full of special abilities. You can't build a synergy gestalt with wizard/sorcerer, because they don't get any special ability worth its name, but on the other hand, a LN druid/monk, or a druid/ranger, or a ranger/paladin, or a monk/paladin, or a monk/rogue, etc. would all be cool.
Here are a few gestalt concepts:
Warlock/Warmage (or Warlock/Sorcerer). With one class having utility/defense incantations at will (plus blasting with eldritch ray), and the other having blasting spells. If you go the Sorcerer route, you can inverse, and use the warlock incantations for blasting, and the sorcerer for utility and protection spells.
Warmage/Hexblade. You fill both sides of the Arcane Warrior concept at once, with fiery arcane power and good skill at the blade.
Scout/Rogue. Max out on Initiative, you may be able to drop your enemies with just one strike in the surprise round.
Scout/Monk. This one requires a bit of explaining: in Oriental Adventure, there is a martial art style that gives you a +10 Competence bonus to Tumble checks, and you can move 10 ft. as a 5-ft. step with a Tumble DC 40 check. With a good dex, and maybe the Skill Focus and/or Acrobatics feat, you can achieve a Tumble score of 30+ around level 11, when full attacks become really interesting. Flurry of blow+skirmish damage...
Sohei/Scout. Same thing as above, with a difference. Rather than trying to qualify for the Martial Art style, you try to qualify for the Bladedancer prestige class. You'll need spellcasting for that. The Sohei gives you Weapon Focus, flurry of blow, and a nifty
ki frenzy which increase your Dex (and thus Tumble skill), Str, and speed.
Rogue/Duelist. What can I say? Sneak attack! Alternatively, Rogue/Ninja, but you'll have to catch your enemies flatfooted.
Finally, thanks to Complete Adventurer, here's a third sort of gestalt, the Complete Adventurer Feat gestalt. The idea is to use of the Ascetic X or Devoted Y feats to have your two gestalt classes synergize each other. They can be incredibly cheesy in gestalt play, since they make levels in both class
stack for the purpose of a few abilities, in other words, doubling your level.
- Monk/Ranger with Ascetic Hunter. You double your monk level for the purpose of unarmed strike damage. And the DC of your stun attacks against favored enemies is increased.
- Monk/Paladin with Ascetic Knight. You double your monk level for purpose of unarmed damage, and your paladin level for purpose of smite damage.
- Monk/Sorcerer with Ascetic Mage. You double your monk level for purpose of your AC bonus, you use your Cha bonus instead of your Wis bonus to AC, and you get a reduced version of the Arcane Strike feat.
- Monk/Rogue with Ascetic Rogue. Double your monk level for unarmed damage, add +2 to the DC of your stun attack when sneaking.
- Paladin/Rogue with Devoted Inquisitor. When smiting evil with a sneak attack, opponent must save or be dazed.
- Paladin/Bard with Devoted Performer. Double your paladin level for purpose of smite evil damage, double your number of bardic music uses per day.
- Paladin/Ranger with Devoted Tracker. Double your damage from smite evil, and give your mount the animal companion's bonus.