States that issue same-sex marriages (green)

Connecticut
In a 4 to 3 decision, the Connecticut Supreme Court overturned the state's ban on same-sex marriage, making it the third state to allow legal gay marriage. On April 23, 2009, Gov. M. Jodi Rell signed into legislation a bill that removes gender references from state marriage laws.
Iowa
On April 3, 2009, Iowa became the third US state to allow same-sex marriage. The Supreme Court voted unanimously in favor of gay marriage. Couples began exchanging vows on April 24, 2009. Here's what gay and lesbian couples need to know before they marry in Iowa.
Maine
Gov. John Baldacci signed a bill legalizing same-sex marriage on May 6, 2009, making the state the fifth in the U.S. to allow gay and lesbian couples to marry.
Massachusetts
On May 17, 2004 Massachusetts became the first U.S. state to legalize same-sex marriage.
New Hampshire
An April 29, 2009 13-11 vote by the state Senate legalized same-sex marriage, upgrading previous civil unions.
Vermont
On April 7, 2009, the state legislature overrode Gov. Jim Douglas’s veto of a bill allowing same-sex couples to marry, making Vermont the first state to allow gay marriage through legislative action. In 2000, Vermont became the first U.S. state to offer same-sex civil unions.

States that recognize same-sex marriages (blue)

District of Columbia
Although same-sex marriage is not recognized within D.C., on April 7, 2009 the DC council approved a measure that recognizes the marriages of same-sex couples wed outside of the District.
New York
Note: By a May 29, 2008 directive, New York only recognizes gay marriages from couples married outside of the state.

States that allow same-sex partnerships or unions (orange)

New Jersey
New Jersey is the third U.S. state to offer same-sex civil unions behind Vermont and Connecticut. The new same-sex civil unions law, which goes into affect on February 19, 2007, grants gay and lesbian couples the same rights as marriage (Although same-sex unions in New Jersey are classified as civil unions, couples are granted the same rights as married heterosexual couples).
Oregon
Under Oregon's new domestic partnership law gay and lesbian couples are eligible for all the state-wide rights and benefits of marriage. Oregon also outlaws discrimination based on sexual orientation.
Washington
On April 21, 2007, Washington's domestic partnership bill was signed into law. The measure was expanded by the state legislature to include all the rights and benefits of full marriage on April 16, 2009.

Source: Ramon Johnson, About.com. Last updated May 6, 2009.