http://lanic.utexas.edu/la/cb/cuba: Hosted by the University of Texas Latin American Studies Department, this site houses a vast collection of information about Cuba. This is hands down the best one-stop shop for Web browsing.
www.cubanacan.cu: Cubanacán is probably the largest state-run tourism company, with hotels, transportation, and receptive tourism arms. They also have the best developed website of the handful of state-run agencies, although that's not necessarily saying much.
www.cubasi.cu: CubaSi is the best of the state-sponsored, all-purpose information sites on Cuba. It's got an English-language mirror of the principal site, as well as a good search engine and easy links to an online version of the Cuban telephone book.
www.casaparticular.info: We find this the best of the clearinghouse sites for casas particulares. It has lots of links to casas in the main tourist destinations around Cuba.
www.cubamania.com: This site is a lively, sometimes conflictive and controversial, forum site for all things Cuba. But there's plenty of good information to be had here.