Player's Handbook II, Book of Nine Swords, and either Spell Compendium or Magic Item Compendium (or if you're DMing, Fiend Folio or MMIII). I am a bit biased, though, since I helped design three of those. I really think they help the game out a lot, though - among other things helping fighters regain some of the territory they gave up to melee clerics, spicing up spells, and most of all adding creativity to the players' side of the table. But I'd also recommend Lords of Madness, DMG 2, Complete Arcane and most things by Rich Baker, a lot of Dave Noonan's work. Both Rich and Dave try to keep the emphasis on fun, imo.