This article from Bunnyfoot covers a range of online prototyping tools available at the moment.
There are four reviewed: Mockflow, iPlotz, Balsamiq and Mockingbird.
I had a play with Mockingbird last month. If anyone has experience of the others, leave a comment.
Web-based prototyping tools reviewed at Bunnyfoot.com
Bunnyfoot are a UK usability consultancy, with an office in Edinburgh.
My university webpage wireframe demo using Mockingbird