From the Top

[Kunimatyu]

Let me echo what other posters have said here:

In my campaign, my alternate "core three", so to speak, are:
Player's Handbook 2
Magic Item Compendium
Spell Compendium

For monsters:
Advanced Bestiary(Green Ronin)
Monster Manual 3 (inventive monsters with appropriate Challenge Ratings)
Lords of Madness/Fiendish Codices/etc. (for whatever monster type tickles your fancy)

Dungeonscape is worth a shout-out if your group likes dungeon crawls; its "encounter traps" are particularly good. Likewise, if ship/aquatic stuff is your thing, Stormwrack is very good.

 

[Razz]

Dungeonscape is very good, Cityscape is...very sad. The sidebars of Cityscape were more helpful and inspiring for my urban-based games than the actual text. Download the DMG Web Enhancement since most of the stuff in there involved in making cities is in Cityscape anyway.

 

[ColonelHardisson]

For players: PHB2

For DMs: DMG2 and Unearthed Arcana (though apparently much of it can be found online in various versions of the SRD).

Manual of the Planes is quite nifty, and is a great book for DMs wanting to tweak their campaigns - the various rules for planes can be used for areas of a campaign world, as well.

Fiendish Codex I was fantastic. I'm not much of a fan of using fiends in a game (I'm not against using them in a game, I just don't have a particular fondness for doing so), but this book blew me away. Lots of adventure and campaign ideas.

 

[Nepenthe]

There's this: http://www.enworld.org/showthread.php?t=196025 thread where people have listed their favourite non-core wizards' books.

I got back into Dungeons & Dragons after 4-5 years last summer and went the way of picking a campaign setting (initially Forgotten Realms, but then quickly reconsidered and moved onto Dragonlance) and after gathering the setting books, started looking at which particular books would help me with that setting...

If you are a Planescape fan, Expedition to Demonweb Pits is probably a good starting point, as is Manual of the Planes... Maybe pick up those two and then start building on those?

/N