If you've been using MS Paint to brainstorm it's time to upgrade. A whiteboard solution called Dabbleboard is one of the better efforts I've seen. It lets you put together a map of ideas very quickly, and supports both free-hand drawing, along with a system that will automatically convert basic doodles into sold shapes like circles, triangles, and squares.
Dabbleboard's killer feature is that it lets you save bits and pieces of these doodles into your library for later use. Once you've added anything to your library you can simply drag and drop it into whatever you're working on, and the pieces will follow you between projects. This is helpful for any complex design elements or images you've uploaded and plan to use in a later session.
In addition to its library tool, Dabbleboard boasts online collaboration that lets you work with several others at the same time. The one caveat here is that only one person can be actively making changes for it to save, otherwise anything you're working on can be overwritten by someone who jumps in and draws something. The system is smart enough to alert you when the other person is using it, however, I found it to do a poor job at respecting the precedence of an edit that had begun before someone else's.
Any work you've done can be shared in a central library with items that can be copied back to your personal collection for editing and redistribution. You can also embed any of these works on a blog or site with code, which means when someone makes a change it will go live wherever it's been embedded.
Besides the standalone site, Dabbleboard offers an API for developers. Anyone can use it to build Dabbleboard into their sites and services, letting users log in and save their work using a pre-existing user account.
The service reminds me quite a bit of Scriblink and Skrbl, two collaborative whiteboard tools I've looked at before. Also worth mentioning is the now-extinct software-based FreeHand, which Adobe Systems killed off back in early 2007. The big difference here remains the clips library, which is just plain smart. In any project where you want to save some time by reusing something you've already modeled, this is going to be a immensely helpful.
I've embedded the service's demo video below.