I'll offer a counter-opinion here. Note up front that I'm not a "joiner," and am more likely to send a money order (checks get you on lists for more donations) rather than be a member anywhere. having said that, and I'm not going to get into a debate as these comments always piss people off:
NRA: 2 out of 3 times I see them engage I disagree with their approach. They do good, but they also do harm, often undermining local efforts in the name of "the big picture," or some indefinable backroom deal that's going down. They're also the kind of organization that pens some of the gun control crap we need to deal with now -- I don't buy into the argument that appeasement is a good strategy. Again, you can flame this opinion if you want, but I won't reply to it.
GOA: I used to be a big fan, then Larry Pratt put pro-life in front of pro-gun for some rankings of politicians. I thought it was still a good organization, but after the "sudden jihad syndrome" crap lately they'll never see another dollar from me.
JPFO: They might be a bit shrill sometimes, but they do good work. Good folks.
SAS: Another good organization. Don't know a lot about them, but like everything I've heard.
Pink Pistols: As with SAS, I haven't seen anything I haven't liked. It's a good idea -- targeting a traditionally liberal segment of the population and selling "CCW and gun ownership are good" by relating to issues they take personally, like gay-bashing.
That's all that strikes me off the top of my head.