If you are looking for a neat little tower defense/action game to play with a handful of friends, I have got a great deal for you. Dungeon Defenders is going to be free to play all weekend on Steam, and they'll be selling the game for half-off of the $15 normal price. That's right, once again Steam steps in to sell awesome games for so cheap that I want to buy it even if I'd never have time to play it anyway.
Dungeon Defenders just launched its latest expansion pack and (more importantly to some) the Mac version of the game today, so you can play it no matter where your computer preference lies. Plus…it's free for a few days! I don't know why you'd need anything more to try it out.
Boot up Steam and take a look. The worst that could happen is you spend an hour or two playing a game that you don't care for, but you might find that you really enjoy getting a group of friends together and tearing down some towers while leveling up and defeating other players. Sounds like a pretty good time to me.