Need to brainstorm with a class or work partner? Browse the web, use an real time white board, share files, etc, etc, etc. With Twiddla, it's all there and ready to go. No need to sign up, no cost if you are using in the basic free mode. No need for users to have email either to enter, just give them the "link" to your board. Great for classroom use, where links can be put in blogs or on web pages for students to participate.
There are some features that do require $$$, but "guest" use is always free. As the Twiddla folks say it's "painless team collaboration for the web."
So go ahead, try it! "Play" in their sandbox first and then start a board of your own (just be sure to keep a copy of the link!)