United States Code http://uscode.house.gov/
State Codes
In practice, you may not have access to Lexis, Westlaw, or Bloomberg Law services, so it is good to know about alternative services as well.
State Bar Associations Provide Free Legal Resources - information on four services that may be available from your state bar association
To see which state bars offer which services, check out this map from the 3 Geeks and Law Blog.