Many states are moving towards some degree of legalization of cannabis.
The following states have legalized both recreational and medicinal cannabis to some degree (with links to the states' website with more information):
Alaska
Arizona
Colorado
Connecticut
District of Columbia
Illinois
Maine
Massachusetts
Michigan
Montana
Nevada
New Jersey
New Mexico
New York
Oregon
Vermont
Virginia
Washington
The following states have legalized medicinal cannabis to some degree (with links to the states' websites for more information):
Alabama
Arkansas
Delaware
Florida
Hawaii
Louisiana
Maryland
Minnesota
Missouri
New Hampshire
North Dakota
Ohio
Oklahoma
Pennsylvania
Rhode Island
South Dakota
Utah
West Virginia
Westlaw also has a Marijuana State Legal Status Overview, which summarizes the current laws of all 50 states in more detail and provides a visual map.
Fox Rothschild, LLP has also created a very thorough National Survey on Marijuana Laws and Regulations, which may be a great starting place for finding the relevant laws if researching cannabis law in a specific state.