I find Dropbox very useful for syncing config files, wallpapers and sharing files with friends. You get 2GB of free storage and can get more through referals. I've found I don't use my memory sticks as much for smaller files. I like that it's cross-platform (Linux/Mac/Windows). There are other similar programs like Syncplicity (Windows only) and Foldershare (Windows/Mac, now owned by Microsoft) that aren't completely cross-platform. For Linux, there's also Ubuntu One which I have yet to try, but it is limited to Ubuntu 9.0.4 and above at the moment only. Anyway this video explains how Dropbox works in a nice simple way, ideal for encouraging friends to join up. :D
Update: Having tried Ubuntu One, it's similar to Dropbox, though it would be nice if it had similar icons to show what files are updating etc, and a distinctive Ubuntu One folder icon. Although you can of course add your own emblems (Edit --> Backgrounds and Emblems in Nautilus) I'll let you know how I get on with it, when I've tested it thoroughly. Also of course it would be great if it was not restricted to Ubuntu, though you can upload/download files through the web interface, but it is not as intuitive as Dropbox's.
Update 2: I've been tryomg out Ubuntu One a bit more, it's ok when it works but at the moment all i get is an icon with an ominous 'X' on it, sometimes it works sometimes it doesn't, maybe it will improve for upcoming Karmic koala, which is not long now.