Dropbox is a service that allows for synchronising files between computers. I've been using it for a couple months now and I quite like it.
The basic operation is that you sign up for an account, install the desktop client on whatever device you want to synchronise files with and then start copying things into your dropbox directory. Anything in that directory will be synchronised with your files stored on the Dropbox server. This arrangement means that you don't need all your machines to be on at the same time and that you can get around firewalls much more easily. The other major benefit is that you can access your files from the web, which is handy if you just need to get at a single file on someone else's computer.
The web interface also has a mobile component (unfortunately intended for iPhone but it works on most mobile browsers), unfortunately it doesn't quite work on the default browser for my phone – I can browse my files but am unable to download them. Thankfully Opera Mini is up to the task and can download the files to the phone.
The synchronisation process is fairly robust and has timely updates when files are changed. You can tell when an update is being done as the icon in the taskbar changed to a "processing" icon. The Dropbox clients for Windows and Linux also place fitting icons on files and folders when viewed in Windows Explorer or Nautilus.
Overall Dropbox is a great system and one I highly recommend using if you need to keep a small number of files synchronised between several computers. For the future I'd like to see some better mobile integration but at least it's not something that is completely missing.