Interesting suggestions. Not sure on non-D&D stuff, but maybe revised Mutants and Masterminds, Werewolf, GURPS.
Games Den I think did good for D&D players, Complete series is very good, I would also add Races Of Eberron and Expanded Psionics Handbook. Maybe they don't play Eberron, but the book has all the rules for playing all the new Eberron races, so you can add them into any campaign. It even has a few sidebars about using them outside Eberron. Spell Compendium would also be awesome, but you might have trouble finding it. Not sure if I'd recommend Weapons of Legacy for a player, getting magic items is kind of depends on the DM, but a box of minis or maybe the Three Dragon Ante card game.
But the DM suggestions was a little weak, Underdark is also kinda old. For DMs I might recommend Heroes of Battle, Heroes of Horror, maybe the enviroment books (Starting with Stormwrack cause its the newest, then Sandstorm, then Frostburn). Libris Mortis if they liked undead (should be easy to tell, cause they'd talk about vampire/zombie movies and stuff all the time). DMG 2 is okay. MM 2 and MM3 could be useful to DM and players, but I couldn't recommend MM 2 because of the older edition and I couldn't recommend MM3 because of all the mistakes in the monster stats.
