After FolderShare from Microsoft, Dropbox is another similar service, but with much better interface and ease of use, currently under closed private beta the service allows users to synchronize files between multiple computers both Mac and PC.
DropBox makes file-sharing and syncing as simple as it can get, The client integrates with the files system and you just need to drop your files inside the special dropbox folder and rest is taken care in background by the Dropbox client. As a file inside the special dropbox folder gets changed, automatic file syncing takes place and online version of the file is updated.
All files inside your Dropbox can be accessed and shared anytime using the online interface, even when your computer is powered down, the best thing about the service is its ability to retain older versions of the file as a backup copy, incase the file gets deleted by mistake from the Dropbox folder.
The service is currently under closed private beta and needs a special invite code to try, Dropbox is holding for few more days to provide more beta codes, once more beta codes are available we will update you.