With the 3.5 Expanded Psionics Handbook, there's no need to pick up the old 3.0 Psionics Handbook, unless you like psions relying on multiple ability scores and psychic warriors having worthless powers. :^D The XPH isn't a great revision in my opinion, but it does fix some of the problems of the 3.0 PsiHB, and has more content, even though it's got a few brokenly-overpowered powers and combos of its own.
The Manual of the Planes is a good resource if you like devising your own planar cosmologies or want decent amounts of detail on the Great Wheel cosmology; each GW plane (including the transitive ones and such) has at least a page or two of description, some of them several pages. The book has several planar phenomena and 'storms' described, plus a few details on things like Githyanki Astral Carracks. There's a variety of optional planes like the Plane of Time, the Plane of Dream, a living plane, a few details on the Far Realm, alternate Planes of Faerie, and guidelines for designing various types of plane or cosmology. It has various spells that are quite useful for planar travel and survival, and use/location/sealing of portals, plus a few neat prestige classes like the Gatecrasher. MotP has various creatures like the Yugoloths, Mercanes, Energons, Inevitables, Githyanki, Githzerai, Bariaurs, and such, plus several Elemental and Half-Elemental templates, the Shadow template, the Anarchic and Axiomatic templates, and the Petitioner template. The Planar Handbook is definately no more than a supplement to Manual of the Planes, more character-oriented whereas the Manual is somewhat more DM-oriented (and it's got better details for DMs regarding the Great Wheel planes).
Personally I think 3.0 Oriental Adventures is a great book (download the errata though, there's a few confusing typos in the book), but it's not really necessary (though it doesn't seem likely that WotC will be publishing any 3.5 OA materials, aside from their occasional insert of a more-or-less-unchanged/unerrata'd OA class into the Complete X series).
The Stronghold Builder's Guidebook, and the Arms & Equipment Guide, are both useful accessories from what I've heard, though I haven't gotten the SBG yet myself. The A&EG has compiled gear, vehicles, tips, magic items, and such from the 3.0 splatbooks (Sword and Fist, Masters of the Wild, etc.), though it doesn't have everything from them (i.e. it lacks their prestige classes for instance), and the A&EG has many good variant armors/weapons/materials as well as new magic items, artifacts, and vehicles. If you already have or are planning to get a few of the class guidebooks though, you probably shouldn't bother getting the A&EG too, since some of its material comes from those.
Of the splatbooks, the ones I'd most reccomend, if any, would be Sword & Fist and Tome & Blood; Masters of the Wild has too much broken junk, and a lot of unhelpful tips and spells, though it has a precious few things in it worth looking at or maybe using. The splatbooks aren't that terribly important though, but do provide a useful variety of options (i.e. S&F's good variety of prestige classes, feats, gear, and fortresses, T&B's similar variety of neat PrCs, feats, gear, and spells, plus item creation/spell creation tips/guidelines).
Savage Species isn't worthwhile, nor is the Epic Level Handbook, nor Deities & Demigods. Monster Manual II is useful though, I think.
