PH, DMG and MM -- used a lot, this got without saying.
Creature Collection 1 -- two of the critters have been already met by my PCs, and they will definitely meet at least 2 others.
Monsters of Faerûn -- Has seen use also, especially by my brother (who's our FR DM).
FRCS -- heavily used.
Psionics Handbook -- frequently used also.
Relics & Rituals -- several spells and magic items have seen uses.
Magic of Faerûn -- used frequently.
Manual of the Planes -- one of the least used in game, but was much useful for shaping my world's cosmology.
Tome & Blood -- spells and prestige class has been used, as well as some magic items.
Deities & Demigods -- probably the least used book I have, although I plane on using the faun.
Faiths & Pantheons -- not much used, but the background bits are nice for our FR DM, as we don't have the 2e books.
Seas of Blood -- not much used for now, but we're supposed to become all pirates in one of our campaign, so it will be used.
Creature Collection 2 -- will see some use, much to my player's dismay, hehehe.
Oriental Adventure -- is actually under heavy use, as we're about to play 10th-level characters in an OA campaign !
I've tried to list them in the order I've bought them.
Books I don't own, but I can get access to them (and from which, sometimes, I borrow one tidbit or another, but usually nothing) includes the other splatbooks of WotC, the Quint. Fighter, Librum Equitis 1, Books of Eldritch Might 1 & 2, Rokugan, Magic of Rokugan, Monsters of Rokugan, and Lords of Darkness. The player who own S&F and DotF has made use of some spells from DotF, after having got the tacit agreement from the DM (who said "these books are too expensive, I won't buy any, however, just in case there's something worthy in it, I allow players to use the material from a book they buy"). Our FR DM was happy when a friend of him lend him the LoD, but except for the updated background, it has seen no use (especially given that it's back to its legitimate owner).
If they weren't so expensive, I would be interested in the BoVD and the MM2.