DROP MOCKS Probably my favorite because there's no signing up or uploading and it's so convenient when all you want is show a quick preview of your work. You literally drag files into the browser window and ta da. Then share the url with others for a quick online presentation. (watch your file size though, it doesn't like high rez stuff)
www.dropmocks.com
CONNECT-A-SKETCHAnother
pointlesscorp app, it basically allows you to upload your roughs & sketches and create an interactive prototype to be presented or shared. I'm still testing this one.
www.connectasketch.com
PREZIShared by a colleague of mine, Prezi is pretty thorough, it does just about any file format, videos, etc... You mix all kinds of content and create your own version of a presentation. The tutorials are pretty helpful too. Start there if you're interested.
www.prezi.com