Our usual gaming group is six players. I think I can hear a groan — we do wind up playing certain games over and over again just because they accommodate six players. We have managed to adapt a few games to allow us to play all together. We've had significant luck with Race for the Galaxy
Dominion
For a quick fix to add players, we add in more victory points. This means that not only is it important to have twelve Province cards available, but we also add in the seven blank cards that come with the game as extra Provinces. It's a bit of a hack — but it works. Games still tend to go pretty quickly: you can imagine how fast those Provinces disappear, but it's not so fast that we don't have some fun in the process.
Race for the Galaxy
Our six player version is really just two games of three players each going concurrently at the same table. We've tried a couple of variations, including using just one set of cards for all six players. I eventually want to see some numbers on how things go down with different variations, but we haven't been tracking closely.
Other Options
When it comes to hacking a game for extra players, there is one important factor: the amount of interaction between individual players. If it is fairly minimal, as in Race for the Galaxy
We're open for discussion on this one: what games have you had good experiences expanding for extra players? Any specific approaches — up to and including playing in teams — that have worked?
I think the expansions for Dominion and Race both scale them to 6.