Two words...

Mario Party

Its a great game series/concept, or at least it was back on the N64 when people actually played it.  Pick any major franchise that's been regurgitated as many or more times and you will normally find the same lack of enthusiasm.

I'm also going to go real old school.  Most of these would take some work to emulate for a console, but even getting to see them again for the PC would be cool:

1. Utopia (ATARI) - This was an old multiplayer strategy game.  Two players each had their own continents and vied for common resources and economic power.  You could sabotage each other too.  It was very simple but very fun.
2. Bard's Tale (C64) - Already mentioned but worth mentioning again.  An NES version was also done and it would suffice.
3. Legend of Blacksilver (C64) - This was a spiritual successor to the more popular and better known Legacy of the Ancients.
4. Seven Cities of Gold (C64) - Excellent colonial strategy game from way back.
5. Montezuma's Revenge (APL2) - I spent countless hours mastering this game when I was about 4 years old.
6. Karateka (APL2) - Always got my butt kicked but it was still fun.
7. Beachhead (APL2) - I thought the recent arcade update Beachhead 2000 with a fully rotating "turret screen" was a good concept.  Needed hugely better graphics though.
8. Wolfenstein 3D, Spear of Destiny, Duke Nukem 3D, Samurai Warrior (PC) - Although not sleepers, all of these would be nice to have as newer compatible releases for XBox Live Arcade.

I'll keep thinking of stuff, but I just thought I'd throw in some really old stuff which weren't likely to be mentioned.

anyone mention Gemfire (SNES) yet?

My cousin and I spent so much time playing that game. He was always the Lyle family, I was always the...uh....the guys that had Zendor.
Chadley10

I just finished reading all of the posts and I was about to mention Gemfire.  I remember really enjoying that game with my friends.

You guys have pretty well covered the list I can think of, but here are a few I remember playing when I was younger which may or may not live up to their contemporaries:

1. Magic of Scheherezade (NES) - sure its a semi-Zelda knock off but it was really well done
2. Sky Shark (NES) - I always enjoyed it but it was pretty hard
3. Ultima Exodus (NES) - I love the Ultima series and I liked the NES version of U3 better than any other
4. Joe & Mac (NES) - really surprised no one mentioned this game because it was pure fun
5. Lion King (GEN) - easily one of the best Disney/Virgin games ever made
6. Spy vs. Spy (NES) - good old multiplayer fun

I'm sure there is more, but its my start till I get a chance to go home and look at the collection for ideas.