I am specializing in conjuration for the first time ever (evocation opposed) and it is a challenge to run with the core rules. The way I've made it work is to plumb the depths of every possible d20 book for more spells, more feats and more prestige classes. I am confident that I have a truly bad ass conjurer as a result. I don't even miss fireball or magic missile. The conjurer's focus is on incapacitation of the enemy (glitterdust, darkhood, power word hesitate, etc.) and on summoning up allies. Quintessential Wizard has two concepts to use: Summoner (buffs all your summoned critters with ability bonuses at the cost of an extra opposition school) and Theologian (gives you a cleric domain granted power and spells---choose Summoning domain which gives you doubled duratrion of all summon monster spells and free acquisition of summon monster I-IX plus Lesser Planar Ally, Planar Ally and Greater Planar Ally in exchange for forcing you to max out Knowledge: Religion skill ranks every level and preventing you from casting spells with an alignment descriptor opposed to your own). You are now on the path to becoming a total badass without needing to multiclass.
Feats: Pick "Augment Summoning" (from Tome & Blood, gives +1 to attack, damage and AC of all summoned monsters), and "Specialist" (which doubles the duration of all your 1st and 2nd level conjuration spells in AEG's FEATS book). Other helpful feats are Spell Focus: Conjuration, Greater Spell Focus: Conjuration, Conjure Mastery (from Book of Eldritch Might), and Spellcasting Prodigy.
Magic Items/Familiars: Quiontessential Wizard has a demon amulet. Get it, add the Familiar power and now your amulet IS your familiar and it lets you cast chaotic and evil spells at +1 caster level and teaches you one extra spell every time you gain a level. You could just house rule the demon amulet into a devil amulet, angel amulet, slaadi amulet, or whatever fits your alignment.
Good Prestige Classes to take: Master Summoner from Plot & Poison: Guide to the Drow. Mindbender (so you can telepathically communicate with your summoned monsters and allies) and Acolyte of the Skin from Tome & Blood. Summoner from Path of Magic. Summoner from Relics & Rituals (reprinted and corrected in Ultimate Prestige Classes).
Now that you've got your character made, use the variant summon monster rules from the DMG so you can roll up your summoned monsters and then "advance" them every time you get a new summon monster spell. Savage Species will be helpful here, as will Diablo II: Diablerie, Armies of the Abyss and Legions of Hell for their wide range of low HD outsiders. You want monsters that advance by character class, or that "morph" into the next higher up version of their creature type, not ones that merely advance by adding only HD.
