Tagalus lets you declare the meaning of Twitter hash tags. There can be more than one version, with users voting up the best definition. You can also add and view tags on its site. The Tagalus API accesses both getting and setting of definitions.
Do you use Twitter, but don't spend your March in Austin? You may be confused by "#sxsw" showing up in your stream. A new site can help make sense of the gibberish "hash tags" spouted by some of the people you follow.