Games that play well with a wide range of different numbers of players
Once again I can't recommend Dominion enough.

The base set supports 2-4 players and really plays the same for all of those, and if you also get Dominion: Intrigue (or a second copy of Dominion, or just some extra copies of a few of the victory cards) you can play with anywhere from 2-6 players.  4 players is probably the "sweet spot" that the game was balanced around, though.

A few basic rules are adjusted based on the number of players - the amount of available victory points and the game end conditions scale with the number of players.

Game times will be affected most, naturally - in a 6-player game you'll be waiting longer between turns.  The other factor that changes is competition for cards - in a 2-player game you will mostly be able to get whatever cards you want, while a 6-player game will require more adjusting of your strategy on the fly to respond to cards going to other players.

All that said, the core gameplay works very well for any number of players from 2 to 6 without excessive special-case 2-player rules or dummy players.