I have used GoogleDocs with a group of authors preparing a book for a publisher. It was very useful for parties in several locations and knowing what was posted was latest and greatest.
We have found a problem with UMCal in that when people schedule an all day event that doesn't show up in scheduler because it doesn't have a start and end time. We have had several coworkers with double meetings because people couldn't see all day events so it appeared their schedule was empty.
Doodle is another useful tool for meeting scheduling with contacts outside the University system.
Created a Ta-Da account and created a To Do List.
I watched the Demo of Zoho and that looks like an interesting site which may come in handy with more organizations converting to MS Office 2007 while we are staying with 2003. I have run into people saying the formating is bad when they open my docs created in 03 with 07. This would eliminate that problem.