Terminus Est needs your input. Please submit in the comments your favorite org(s) that are fighting to guarantee marriage rights to all citizens, regardless of sexual orientation, and why you like them. The Wikipedia article on Gay rights organizations lists like 114 orgs in the US alone.
I already donate to Immigration Equality, which puts effort into reforming immigration law so that people can sponsor their LGBT and HIV positive partners for green cards, which, even if you live in a sate with legal gay marriage, currently you cannot do. Now I’m looking for orgs which are working for marriage equality.