Games suitable for largish groups of players include:

Take It Easy. Interesting game. It's really more of a simultaneous multiplayer solitaire puzzle than a game. It's fun, but playing and enjoying it a few times last year didn't make me want to run out and get one.

Shark Decent Acquire-like game; it's one of the few Acquire-inspired games to borrow the touching property aspects of Acquire. 6 is pushing the limits of the game.

Medieval Merchant. I've only played it once; so far, so good.

Razzia. One of the very few 6-8 player fillers, and very useful for that reason. Not a great game, but enjoyable.

Tutanchamun Very good game - one of Knizia's better ones, but we've never played it w/ 6.

Katzenjammer Blues Yuck. Nice idea, decent mechanisms, but it ends just as it's starting to get interesting. Double-deck might help.

Lowenherz (yeah, yeah -- I know this only plays for 4) Very good game (though it's SdJ competition and winner Mississippi Queen is a bit better, and does handle 6 players w/ Black Rose). One of the games I won't play with anything other than 4 players, but good enough to stick around anyway. Just get past the basic game rules as quickly as possible - I'd even suggest setting up the basic game, playing a few rounds to get a feel, and then starting over.

Isel Argern Good game; you definitely want to play with a variant or two with 6 players to keep things moving.

Attacke Yuck. Another Knizia klunker (as opposed to Rosenberg's game Klunker, which is supposed to be quite good). We tried it twice, just to be sure, and it hit my trade pile shortly thereafter.

Power Play - Previously marketed as Phantoms of the Ice. Any changes from the Phantoms incarnation? A few additional special cards. Any good in either case? Quite a lot of fun, at least with 4+ players. It's light and airy fun, though; if you want deep strategy and the like, try something else.

I'd also suggest Carabande, Pit, Great Dalmuti, Union Pacific, Ave Caesar, Bohnanza, Business (a.k.a. the game that really should have kept it's original name - Plan Ahead), Elfenland, Der fliegende Hollander, Der fliegende Teppich, Hattrick, Montgolfiere, some of the New Games in Old Rome, 6 Tage Rennen, Showbiz, Showmanager, Sticheln, Take 6, Time Agent, Ursuppe, and Viva Pamplona.