Expanded Psionics Handbook -- I love Psionics, but almost all of the information is available
online, and the online version has all the errata in already.
Tome of Magic -- I love the Binder, and I like the other two classes, but they are underpowered and non-trivial to fix, and also non-trivial to integrate into a "regular" campaign. Excellent value as inspiration for a world-builder or tinkerer; not much value otherwise.
Unearthed Arcana -- Good stuff, but also available online (same place as Psionics, but in the "Variants" section). If money is an issue, don't buy this book.
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Tome of Battle: Book of Nine Swords -- Fun fun fun new system that integrates well with D&D and makes combat fun for melee guys again. I love this book.
Player's Handbook II -- Nice new classes, good selection of new spells, affiliations are cool. One of the best 3.5e supplements ever.
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That's really it. I haven't gotten tons of value from the Spell Compendium -- there are enough bad things in there that it's been work sorting through what's merely good vs. what's seriously broken. No opinion on Magic Item Compendium yet.
Cheers, -- N