Splash, an event planning/promoting web app, is so great looking and easy to use, I immediately planned a party, just to play with the platform. For free, users can add widgets like music, speakers, sponsors, and maps to their event page. You can also prominently feature an event hashtag and track social activity on the site, and at your event.
Here are some of the other great sites I discovered at SXSW:
Film Maker Live: A great way for filmmakers to test locations digitally without the cost of setting up a shoot. http://filmmakerlive.com/
Event Base: The creators of event apps for SXSW, Lollapalooza, Sundance, and London Olympics. http://eventbase.com
Creative Live: Thinking differently about education, offering all sorts of courses, from business courses to design and music production. http://www.creativelive.com/
Jack Prints: Promotional items with sustainability in mind. http://jakprints.com