Gamma World was a fantastic RPG that focused more on fun at a time rather than Byzantine rule, and was often used to test things that would often go into Dungeons & Dragons. Yet it was poorly presented and got little advertising, looking a lot worse than other games at the time. And Wizard is bringing it back in December.
Deep7 games: They created "beer and pretzel" systems designed so you could roll up a game and complete it in an hour. They have a High Fantasy setting (Arrowflight) that reminds me of a less Grim Dark version of Dragon Age, and they even did a Red Dwarf role-playing game with a riotously funny sourcebook! It's too bad they folded when one of the designers passed away from cancer.
It would seem that they are back in business, because their website (worth a look) is still willing to sell product.
Traveller: Has an extraordinarily crafted Backstory in which you can just bathe. Tons and tons of separate cultures, a history that goes for thousands of years. A sophisticated political, economic and social system, and the potential for stories from fabulous epics to minor one-offs. And D&D is the only RPG most people have heard of.
The Tomb Raider Trading Card Game. What's that? You've never heard of such a card game before? That's because it only ever released three expansions before the company went bust and dropped the franchise. However, the cards are fairly easy to obtain on eBay (though check the American site if you are in the UK), and the game is very fun and interesting to play. It can be played single player, but for best results grab a friend and race/fight for the treasure. It's also surprisingly good as a spectator game.
DOOM: The Boardgame is an extremely fun tabletop game for you and a couple friends. One player plays the demons ("invaders" as their called in this game) and up to 3 other players are marines trying to escape. The number of monsters increases with each player added to the game, and the marines have to work together to survive. Despite it taking its visual cues from DOOM 3, it feels closer to the fast paced originals.
Almost everyone who has played a game of Heroscape would agree that it is a tactically deep and absolutely awesome game. Alas, Wizards of the Coast acquired it and promptly stopped it. Since then, most of the community has faded away.
In Nomine is a unique experience with a setting flexible enough to play just about any kind of story you desire.
Prose Descriptive Qualities system games, especially Dead Inside and Swashbucklers of the 7 Skies. Early games like DI are still fairly rough around the edges, but S7S is the most recent, very polished version of the game. It's lightweight and easy to play, and a knowledgeable Game Master can get complete newbies ready to go inside an hour of first introduction for most PDQ games, complete with character creation.
Nobilis is a very obscure freeform diceless RPG wherein the players play as Anthropomorphic Personifications of elements of the world, like Fire, Recycling, or Dreams, anything that exists in the world. Despite the extremely high power scale of this game (all Nobles can warp reality to extremely frightening degrees), most of the conflict of the game rests upon social politicking, philosophical discussions over the nature of the world, and finding ways to harm your opponents in indirect ways. The game discourages using brutal force or killing your opponents outright, which can be very unintuitive given the immense power a player wields, and probably goes against the typical mentality of powergaming RPG players. This, coupled with a setting that is difficult to get into and the author's sometimes confusing writing style, results in few people knowing about it, or understanding what it's about.