Recommendations for 6-player board game
Hands down, Power Grid is fantastic.  Whenever our gaming group has a game night, Power Grid gets played at some point.  The nice thing is it comes with two different boards, and if you get tired of those, there are expansions for around $10-$15 that come with two new maps and rules.  Once you get the rules down, even with six players, we've found it plays incredibly fast.

If you get the Inns &amp; Cathedrals expansion for Carcassonne, you can play with six players, and Carcassonne is another personal favorite of mine.  It's fun to watch people figure out how to sneak their meeples into a city or farmland for the first time.

If you want something lighthearted, sets from the Munckhin series such as The Good, the Bad and the Munchkin are fun for playing, and have plenty of interaction between players.