Couldn't make up my mind so I joined NRA, GOA, JPFO, Second Amendment Foundation, CCRKBA, and LEAA.
Now I've cut back (to save $$) to the first three, although all are worthy organizations. I am tending to give more $$ to JPFO these days.
LEAA is geared toward law enforcement issues mostly, but strongly supports civilian issues such as concealed carry laws, victims rights, citizens right to self defense etc.
Of course now there is also SAS, Pink Pistols, and a slew of other national organizations, as well as tons of local ones.
If you can't afford to donate might I suggest other ways to contribute- I have a credit card that donates a certain % of purchases to Second Amendment Foundation; my long distance company contributes to (I think) JPFO and so does my ISP. So by getting stuff I would have to pay for anyway, I can contribute and not really miss the money. Most of their websites give info on how to hook up one of these deals.
You can even contribute to NRA and GOA through payroll deductions via United Way.