One that hasn't been mentioned is Twintris, in my opinion one of the best Tetris games made. As the name implies it has two playfields and is designed for head to head play, but plays excellent as a single-player game as well.
About online gameplay, I think it's great that games can be played online. But not at the cost of playing with visiting friends on the same machine. Several recent games have gone that route, Colin McRae Dirt for example has no local multiplayer capability but does have an online mode. It should've had both in my opinion.