Well, it looks as though they are having some site issues at the moment, but I've heard a lot of good things about GTGE:http://www.goldenstudios.or.id
I can also recommend JGN (Java Game Networking), a networking library meant specifically to simplify the process of game development with networking and it even has a module for use with GTGE. However, if you decide to use JGN but end up with a different 2D library that JGN doesn't provide let me know and as long as it's even modestly popular I'd be happy to make a connector module for it. However, it does also support Swing if you decide to use it.
You can get more information about JGN here: http://javagn.org/