Dropbox - Simply Awesome

1. Easy to use. Looks just like any other drive on your system. ~/Dropbox (by default.)
   Files are always there, even when not connected to the internet. Only syncs when connected to the internet, and the growl notifications are really slick.
   
2. Drop something in your public folder and make it available easily with right click "copy public link" - send the link out in an email. Really nice for sharing large files or maybe posting an file or image that you want to link to in a forum post or email list where you can't add an attachment. Can even act as a mini static webserver for quick previews of an html/flash site - just give someone the link to a folder in your public dir.
   
3. Works as a mini-time machine/backup. Keeps backups of versions of your files.
   
4. Easily create a shared private directory. This is really great for work or collaboration between people (probably great for contractor communication with a client on screen shots etc.) No need to use sharepoint (blah) just throw in your uml diagrams or project notes into the dir and everyone it's shared with gets notified and gets a copy whenever their online. (It actually has some versioning as well, so acts like mini vcs.) This one guy at work has created some nice unix scripts that he updates frequently... now I just share a directory with him on dropbox, so as he makes changes to the scripts, I have them (I just added the shared dir to my system path.)
   
5. Great for keeping files in sync between different computers. I put my 1Password file out there and now my wife's 1Password app and mine all share the same file (encrptyped of course.) Can be great for sharing any files that you want across systems.
   
6. ***iPhone app*** awesome. GREAT PDF solution. A lot of iphone people complain about not being able to keep PDFs on the iphone - their solutions are all pretty lame (email storage.) Well, with Dropbox you have access to ANY file in there. Great place to store pdf books you want to read using the iphone.
   
7. Cross-platform. Nice to have a client for Mac/Windows/Linux
   
8. 2GB free storage is pretty nice.