d20 product suggestions?

The "Complete..." books are all good. Complete Warrior, the first one, is especially nice, since every character gets into a fight sooner or later. They're all nice for new character options. The smaller books were (clearly) designed for a few classes specifically, and so their use with other characters was extremely limited. The "Complete" books, however, find applications for all classes. "Complete Arcane," for example, has arcane character material for all characters with even the slightest inclination toward magic, as well as new base classes. The art is nicer than in the old splatbooks and they are generally more useful. They really manage to find a little bit of useful material for everyone. There are enough new spells, prestige classes, equipment, magic items, and feats to find something you like.

The Manual of the Planes, for as much as it is maligned for ignoring Planescape concepts, did adequately cover planar environments (if not locations and culture) in 3rd edition. The Planar Handbook has some interesting things, but I prefer Planewalker's material, personally. Although planar domains are cool.

Monte Cook's stuff is always good, and usually pretty low in price if you get it electronically. In particular, Anger of Angels, Beyond Countless Doorways, and the Books of Eldritch and Iron Might (respectively) are very nice.

I like the line of books (I don't know what to call them) that include the Draconomicon, Libris Mortis, and Lords of Madness (though I would have preferred the old title "Codex Anathema" if for no other reason than to retain consistency in the "ancient tome" title theme).

I was never too interested in the environments books. Either the campaign is "like normal, but this time everyone lives in the tundra!" (too gimmicky) or there aren't enough times to use the rules in it to make it worthwhile. But that's just my general impression, and admittedly without much actual experience with the books to back it up.

I'll thoroughly second that. Pages upon pages of fluff and flavour with clean, crisp formatting and carefully chosen interior art (rather than the WtC angry-fruit-salad-that-attracts-twelve-year-olds approach. I-like-hyphens.). Would that I had (a) the means to buy it or (b) a credit card to get it ee-lek-tron-ik-ly.

I would recommend getting more of the Planescape boxed sets over any of the new WotC supplements, many of which are simply filler designed to keep them in business. Planes of Chaos and Planes of Law had some of the most interesting locations in the planes, and some neat mini-adventures as well. Track them down on Ebay, they're worth the price. Well, maybe not $150 for Planes of Conflict, but still.

You can read and re-read the boxed sets, and use them in practically any version of D&D or fantasy gaming ruleset. Pure gold.