Note that the maps all have preview images, but they can take quite a while to load.
Mods
General mods
Note that there are more mods than I've listed here. These are just the ones I thought well-suited for MP games. Check out Sombre's Mod List on the Shrapnel forums for a much more complete listing.
Nation mods
It is worth noting that this list of mod nations is very far from complete, and consists simply of those mods I personally feel are well balanced for MP. You should check out Sombre's Mod List to see many, many more awesome and well-made nations.