Doug McCrae said:
I've got the DnD 3.5 core rulebooks - PHB, DMG, MM. After that what are the best or most useful products for DnD?
I was thinking of getting Eberron, maybe Beyond Countless Doorways cause I'm into extraplanar stuff (don't like the Planescape setting though).
If you're into extraplanar stuff, the Manual of the Planes is a good buy. Do not consider is as a guide about the Great Wheel, it's really a toolbox for creating your own cosmology and understanding how planes and magic interacts (which spells relies on which planes, like
teleportation and the astral, and how you use them when you leave the prime). It's a good buy for Eberron (most planes in Eberron can be likened to one, or sometime two at once, in the great wheel, so you can use the example material easily), and it's a good buy for Beyond Countless Doorways (BCD gives you lots of example planes and a few advices and rules to build others, but that's a companion to the MotP).
Eberron is a good buy if the setting appeals to you. Some love it, some hate it, others are mostly indifferent (like me). It's always good to have a detailed campaign setting as a starting place, even if you merely use it to help you build your homebrew and you end up never playing in the setting.
Other than that, and depending on the type of game you want to play, you can take a monster book or two (Tome of Horrors is a good buy, although it's not up to date with 3.5, because it contains lots of classic monsters and helps you diversify the ranks of outsiders, feys, and dungeon-delving creatures, from WotC the most interesting monster book for now is the Fiend Folio, which is rather 3.3, but the MM3, if you can get it, will be perfectly 3.5); or you can take the Expanded Psionics Handbook.
If you think of buying the Complete Thingie and Races of Stuff collection, wait a bit before. Gather informations on the web about the new classes and races they introduce, and ask yourself if you would want to use them, and how they could fit in the setting. Look at the excerpt on wizards.com, and read a few reviews to get a feeling of the content. I know I won't use any of the Complete Thingies to date, so I won't buy them.
Finally, a last thing you may want to invest in (if you're rich), is the WotC miniatures. They provide a nice visual help, since, with a bit of luck, you'll get monster minis that actually look like the monsters you use; and they provide a nice tactical game to play while waiting for all the players to arrive (I don't know in your group, but in mine, some people are
always late).