Gay marriage, as opposed to civl partnership, is currently legal in six states if you count Washington DC, though this applies to the US capital rather than the state of Washington. The first to legalize it was Massachussetts in 2004 and the others, in order of legalization, are Connecticut, Iowa, Vermont and New Hampshire, with Washington DC as the most recent (2010). New York and California both recognize gay marriage (California in certain circumstances only) but don't perform them. So two gays can't get married in those states, but if they have a certificate from another state they will be given the rights and standing of a married couple.

In addition, there are six other states that allow civil partnerships, which confer most of the rights of marriage, and both Hawaii and the District of Columbia have some legislation on gay partnership rights. You can find a full list here.