There are quite many choices if you look around, in uni we used http://cosketch.com for math alot and i was very happy with it, it looks a bit outdated today but the workflow is better than most similar tools i've tried, very quick, non intrusive, keyboard shortcuts etc. Only drawback is that things are not saved permanently and it doesn't have many widgets if you plan to use it primarily for mockups.

Another good tip, if you trust your friends, is just starting a remote desktop or teamviewer session. Then you have full access to proper tools like Word, Photoshop or IDEs.

I'm working on something similar: - vector graphics/drawings created by freehand / writing http://write-live.com/d/dba21681-8d3f-4fbe-8b4b-e5c1983df934

http://write-live.com/d/8f9b7846-a7b9-4e5c-b704-dad9aa87d14e

- unlimited* levels of zoom http://docs.write-live.com/WriteliveServer/webview.html?d=34...

- draw on a tablet, view on tablet / web http://write-live.com/d/538254c5-7d31-41f2-83bb-bcd0a7cee7ab

A bit like if google docs had a baby with OneNote

I finished something like this too, after working on it for about a year: http://papertogether.com/ Of course it's not as fully featured, but it was a fun experience. I'm surprised to see that so many others had this idea and implemented it too - I thought I was the only one interested in collaborative drawing.