Google Docs is of course well-known. But maybe like me you were aware of it, tried it a while ago, and then didn't use it for some time. If so, have another look. It's truly brilliant now. You can upload an Excel workbook and retain all the sheets, and most of the functionality. And of course, in addition, you can use it collaboratively, and it keeps a change history. I've been using it on a project for a client recently. My client is an accountant. He described it as 'cool'. How often do you hear an accountant say that about anything? ;-)
I've also been using Mind42 for a little while now. I've got a few pretty hard-baked developers hooked, and the accountant I just mentioned likes it, too. There are a number of mind-mapping programs, such as Mind Manager and Freemind, but as far as I know thay are not SaaS and not collaborative. Mind42 is. It's only real weakness, for me, is the lack of a facility to relate concepts laterally by drawing arrows.
The more I use collaborative tools the more I like them, and the less I like the 'old way': emailing versions around and then losing track of them.
Do you agree?