A while back, I reviewed Dabbleboard and recommended it as an easy virtual whiteboard. Here's another great Web-based tool for collaboration. Skrbl allows you to create a space and invite others to view and modify what you are working on.
As it says on the website:
Sketch, text, share files, upload pictures all in one common shared space. There are no new tools to learn, nothing to download, nothing to install.Here's a link to try your own skrbl space.