American Go Association: The official US Go organization. Get the latest US Go news, find out about local tournaments, or sign up to become an official member of the AGA!
Goproblems.com: A great site that lets you view and post Go problems. Spend a significant amount of time here and you are sure to become stronger.
Kiseido Go Server (KGS): This is the major US Go server that allows you to play online. They also publish books and sell Go equipment. At any one time, there are hundreds of players from around the world playing on KGS. They also sponsor lectures by pros and simulcast major US Go competitions.
The Internet Go Server (IGS): The largest Go server on the web. IGS has more of an international community. Similar to KGS, players of different strengths from all over the world play here. They also simulcast major international competitions (including Meijin, Honinbo, Kisei, Judan and Tengen titles and the Ricoh Cup professional pairs tournament).
Sensei's Library: A Go wiki site! Contains information concerning anything and everything about Go. The folks that post at this site are incredibly knowledgeable and pretty much cover every Go topic possible.