PocketMatrix

I heard that has multiplayer capabilities. But I don't know if there is an active user group. If you like RTS there is which had 2-3 multiplayer games a night during the summer. If you go to the forums you can get info on a few that are usually willing to play (like alfelker or verdagon).
*edit*
There is also which is a modified version of PQ that only allows pda users to join. They have or will have a dedicated server for the game and there has been talk of a number of members (pdai.org) getting together to play.

I think nearly all of Doug Beck's games have multiplayer capability. Unfortunately, there was no-one online the last time I tried RocketElite.