The legal landscape of same-sex marriage is ever-changing. As of March 2010, Connecticut, the District of Columbia (D.C.), Iowa, Massachusetts, New Hampshire, and Vermont allow same-sex couples to get married and enjoy all of the benefits of marriage provided by the state. California recognizes the marriages of same-sex couples if the marriage was performed in the four or so months between the Supreme Court's approval of same-sex marriage and the voter referendum that took that right away (from June 16, 2008 to November 5, 2008). California also recognizes the marriages of same-sex couples who married in a state or country where it's legal if the marriage was performed before November 5, 2008 (marriages performed on or after this date get the couple marriage-like benefits, but not the use of the term "married").
Other states recognize marriage-like relationships that provide varying levels of benefits to same-sex couples. These include domestic partnerships, civil unions, and the like. To learn more, see Nolo's article Same-Sex Marriage: Developments in the Law.