I've been thinking today about what I want game I want to make after Stompy. I'm thinking that it will be a space adventure game, where you go around and shoot stuff and get items. The catch is that you can go into space arenas, hook up to the internet, and fight people online. Since this would be my first online game, it won't be some "uber 1337 MMORPG over teh interwebs". The online portion will simply consist of a one on one duel mode against another player.
I've been looking for a cross platform network library for this reason, and because I'm probably gonna add an online high score to Stompy. The one I'm looking at right now is HawkNL. It looks good, and it has a devpak to go along with it.