EDIT: Beta is over. Dropbox is released! So I'll turn this into an "Impressions" thread instead...
I've got 10 dropbox beta invites if anybody wants to give it a try.
In a nutshell, Dropbox creates a folder on your system (Windows, Mac, Linux) and files you drop into the folder get "synced" with your Dropbox Online. You can keep multiple computers synced up atomatically if you install Dropbox on all of your machines or you can access your Dropbox via the web interface on machines that do not have Dropbox installed.
You can have shared folders with other Dropbox users and you can also send links to files to people that do not have Dropbox accounts. Shared folders allow multiple collaborators to make changes to files and every change is tracked.
Dropbox syncs up with your files online by sending delta data only so large files can be updated quickly. Dropbox also tracks all revisions and deletions so you can very easily go back and restore old versions or files or just extract data or whatever.
Dropbox has a Photos folder that lets you simply drop images into and they instantly go to an online album for sharing.
When it goes live you'll get 2GB free but can buy more space if needed.
Anyway, it's pretty slick. I've been testing a bunch of online syncing services and Dropbox is one of the better so far. I also like Syncplicity.com but they do not have Mac support yet.