I've been using Dropbox for several months now and I love it!
I originally heard about Dropbox on Lifehacker as a tool to sync data between systems. Since I recently got a netbook, I needed a solution to sync data between my different systems.
I couldn't be more impressed with Dropbox. My favorite thing is that is just works. It's not something I have to monitor or manually do. Instead, anything I put in the Dropbox folder gets automatically synced between my systems.
The other feature that I really like is the Public folder inside of Dropbox. It basically gives me a public URL to share large files with anyone.
Signup here and we'll both get an additional 250MB free. https://www.dropbox.com/referrals/NTMzNjkzODg5