On Linux, after downloading and installing the program, you just have to start the daemon:
During the set-up process, a folder is created in your home directory called "Dropbox" and everything you put in there is automatically synced with all the other computers you have linked to the account. You can explicitly share folders with others, or place files in the "Public" subdirectory and then share the associated url.
$dropbox start -i
I have started using this service as a virtual flash drive and as an alternative to emailing enormous files.