Available at http://www.ncbi.nlm.nih.gov

Genbank is the database for genetic information setup by NCBI.

Through a very simple web-based interface, it allows you to search all known DNA and protein sequences for similarity to a submitted sequence.

The electronic searching of genetic information, through Genbank and other similar databases, have revolutionized the Biological Sciences.

Personally, for something set up by the government, I think it works remarkably well.