With TripleA you can play in the TripleA Online Lobby. Simply press the "Play Online" button on the TripleA start screen.
TripleA War Club
TripleWarClub has Online, Play-By-Email, and Play-By-Forum ladders and tournaments, and is one of the best way to find opponents.
Axis & Allies .org
AxisAndAllies.org is a website dedicated to Play-By-Forum only, as well as strategy and boardgame discussion, and is a great place to find opponents as well.