The marriage-equality organization Freedom to Marry has a roadmap showing their plan to get to full nationwide (US) marriage equality.
That link is a very high-level overview, though. They also have a four-page PDF giving more details; among other things, they're hoping to “add four states by 2013/2014 (Hawaii, Illinois, New Jersey, and Oregon)” and six more by 2016.
I don't have a sense of how good an organization this is to support; I know, for example, that their decision not to put money into North Carolina last year was controversial. Anyone have any thoughts or opinions about them? I'm inclined to donate to them, but if there are reasons not to, let me know.
Meanwhile, here's an article from the Advocate about which states are next. This was published in June, so it's already slightly out of date, but it's got a bunch of good info.