Unfortunately, I don't own very many at all anymore. Had a whole bunch up until about 8 years ago. A good friend of mine still has most of the stuff we played. Here's what I can recall without too much effort that we have owned/played over the years (and its seldom we played without owning):
From TSR/WOTC: D&D (Basic, Expert, Companion levels), AD&D 1st, AD&D 2nd, D&D 3rd editions (participated in Greyhawk, Spelljammer, Forgotten Realms, Mystara, and several home-grown campaigns). Gamma World (2nd and 3rd Editions). Top Secret, Top Secret S.I. Star Frontiers (rocked IMHO).
FROM ICE: Rolemaster, Middle Earth Role-Playing (MERPS)
FROM STEVE JACKSON: GURPS (Horseclans, Black-Ops, Fantasy), Car Wars
FROM MISC: Recon, Guerilla (old photocopied rules from 1976), RIFTS, Mechwarrior (Battletech is NOT RPG in itself), Star Wars RPG , Amber, Star Trek RPG, Twilight 2000, Warhammer Fantasy RPG, Shadowrun, Warzone RPG
Lots of stuff that I can't recall, either. One thing I would like to point out, as its a pet peeve. An RPG is a Role-Playing Game, not a miniatures game. Nothing irks me more in gaming than people calling miniature games RPGs. Nothing personal to those who've listed them, but Battletech, WH, WH40K, and Star Fleet Battles are not RPGs. They have variants that you can use in conjunction with them, but by themselves, their just wargames. If I counted the wargames I played, the list would be even longer still.