The GOA is the first gun group I joined. They don't take no for an answer. Small compared to the NRA but very vocal. The head cheese, Larry, is fun to watch/hear in a debate with the anti's as he always wins. They send out newsletters, alerts and pre-written post cards to send to congress.
That's all I can think of for now.
As an aside, I joined the NRA this year just to help out with the numbers but I have found they have a nice monthly magazine that is included with the membership.
Heck, join them both.