Put your files into your Dropbox on one computer, and they'll be instantly available on any of your other computers that also have Dropbox installed. Because copies of your files are stored on Dropbox's secure servers, you can also access them from any computer or mobile device using the Dropbox website.
You receive 2 GB of storage space for free, upon registration. You may earn up to 10GB of free storage space by referring new users to Dropbox, at 250 MB per referral. For $10/mo. you get 50GB of storage, and for $20/mo. you get 100GB.
A list of 3rd-party apps have Dropbox integration: https://www.dropbox.com/apps
Cloudfiles offers Dropbox syncing on Windows Mobile, see http://alternativeto.net/software/cloudfiles.
Unofficial portable versions of Dropbox:
http://www.techbeta.org/portable-apps/dropbox-portable/
http://dropportable.ho.am
