::examines shelf::
Well, let's see.
I'll second MythandLore on MotP and the FRCS. The Manual of the Planes is, imo, one of the best books WotC has produced for 3e so far - if not the best. And it speaks highly of the Forgotten Realms setting that I consider it useful even for my homebrew world.
I'm also going to make a bold statement here and say that I've found the classbooks very useful in my games. Sword & Fist, Defenders of the Faith, Song & Silence, Masters of the Wild - all good. But Tome & Blood is my personal favorite of the five. You may also want to consider the Psionics Handbook, but (obviously) only if you plan on including psionics in your game.
If you're willing to go third party (and I assume you are since you already have) then I cannot recommend Legions of Hell enough. Staplebound, $15, nicely illustrated, packed full of devils to fight. By far the best third-party supplement I've seen. On the other hand, it might not be quite appropriate for your campaign, I don't know. But I recommend it anyway.
Other than that, nothing really leaps to mind. It looks like you're on the right track already with the Creature Collection - I've also enjoyed the CC2 and Relics & Rituals. Out of the HBG, I loved the alignment test and the background tables, and thought the rest was okay, though aimed more at new gamers (which your post implies that you might be).
The only d20 book I've actually regretted buying was Dragon Lords of Melnibone - I love Moorcock, but this book did nothing for me.
For that matter, I'm currently reading through the Pendragon rules and I regularly buy GURPS books just for the background information. You, on the other hand, may want to use a little more discretion when it comes to spending your money.
Of course, I'm biased towards what I like. You should probably check the review page to get a feel for what's good. And a little better idea of what you're looking for would help us as well.