Dropbox style file sharing.

[Arbu]

I have a web application which allows the user to have an account and store files under it. I'd like him to be able to share those files with other registered users, in a similar fashion to that done in Dropbox, except that he would never have to upload anything - the files are just created in the application. Can anyone give me any pointers as to how to do this? I guess it may not only be Dropbox that does this, so there may be a library or existing code that I can use.

Thanks.