Someone has already mentioned Dropbox, and I want to emphasize how WONDERFUL it is. I am the "accidental techie" for a statewide nonprofit with a staff of about 13, including interns and contract consultants, based in two different cities. We started using Dropbox (the 50GB version) for file sharing in June, and it's been a life-saver. Everyone downloads the client to their computer, and we all have the latest versions of our shared documents, spreadsheets, photos and more no matter where we are geographically. Because Dropbox saves files to your hard drive, we also have them when we're offline. I back up the entire Dropbox to a hard drive every couple weeks just in case, but I've never heard of data loss from Dropbox - in fact, we've used it to recover corrupted and accidentally deleted files.
Before this, we always e-mailed back and forth (we had a pseudo-server set up in the office, but it was iffy), so we had many versions of everything. Now, we always have the latest version. The only things we don't use Dropbox for are sensitive files (we keep those on our individual hard drives) and files that we update simultaneously.
For real-time file collaboration, we use Google Docs, having migrated to Google Apps Education edition a couple months ago. We've had the same experience as others in this thread - as word processing software, it's not great, and printing is terrible, but it's nice to be able to edit simultaneously.
Switching from our old, clunky, non-html-supporting webmail to Gmail has been transformative as well. Most of us still use POP3 to manage our mail in Outlook or personal Gmail addresses, but for the two people who were still using the old interface, it's made a huge difference. It's also nice that I (as system admin) now have control of users and can reset passwords when others forget.
Several of us use Google Calendar and like it, but we have not tried syncing with Outlook yet.
The transition to Google Apps itself was rushed, so a little frustrating. After a couple weeks, though, everyone was happily using Google Apps. If you decide to go with Apps, I recommend having a few training sessions with email, calendar and docs before you redirect the MX records and make it live.
I hope that was somewhat useful. I have never used BPOS. Feel free to contact me though, if you want to discuss more - johnson at cedam dot info.
-Katie