I can't believe no-one has mentioned Spartacus-A game of blood and treachery. It is a relatively easy to learn game with tons of social interaction, as it encourages building alliances and back-stabbing at the same time. Of course, for the same reason it is not recommended if your group gets into fights over games. Definitely my favorite board game.
Spartacus also has enough randomness in it and many different winning strategies; both features greatly improve replayability. I've played it dozens of times and every time was great. The base game is for 4 players max, but with a relatively inexpensive expansion you can have up to 6.
Battlestar Galactica is similar in a sense that it also has back-stabbing and betrayal, although this is handled in a much more subtle way. I've only played twice, and we haven't gotten all the rules right (yet). I found the rules a bit too complicated, but I am sure that with time you can master them. Another disadvantage for me is that each game lasts 3 hours or so, which is a bit too long for my tastes. Having said that, I love the idea of the game, thematically. If you and your group loved the series, you'll probably love this one too.
I own Small World as well and I highly recommend it. It doesn't have as much social interaction as the two above mentioned games. For this reason, I would prefer Spartacus over Small World. Nevertheless, because of its deceptively simple base mechanic but its incredible depth, its rather short playing time, its lighthearted theme and its funny artwork, it is a great game for casual players.
EDIT: I also heard good things about Lords of Waterdeep but have yet to play it.