Lot of buzz lately about Dropbox, and I gotta say, it is NICE!
Available on Linux/Mac/Win, it adds a Dropbox folder that is automatically synced securely to the cloud. Set it up on multiple machines and you have immediate/automatic syncing of files/folders etc etc. You can also selectively make files/folders generally public or available to specific users. You get 2GB free, else $10/month gets you 50 GB or $20/month gets you 100GB.
Check out Maximumpc's entry on 15 cool features too for some added info.