Tsuro - easy to learn and scales well from 2-8 players. There is only one action per turn, so you don't end up waiting forever with a larger group. That said, the game mechanics aren't very deep, so it probably won't be engaging for more than 3 sessions.

Kill Dr. Lucky - the classic game Clue, in reverse. The party guests all have personal motivations to bump off the hapless Dr. Lucky, but have to avoid being seen doing the deed, and the old man always seems to slip away. Easy to pick up, a fun twist for people who have played Clue to death, good for 3-6 players.

Bargain Quest - if your friends are interested in something more complex, this game is a lot of fun. You are a shop owner in a classic fantasy RPG world, trying to attract would-be heroes to buy your wares. You can try to help the heroes by getting them to buy things that will genuinely help them... but at the end of the day, you're not crazy enough to go fighting monsters yourself, you're just trying to make a living. And if the hero dies, well... there's always another nut who thinks they're on a sacred quest. Technically this can be played with 2, but I think it needs at least 3 to be enjoyable. Slow-deciding players can make this not fun to play, especially with 5-6.