Take a look at the Wiki article on symlinks: http://en.wikipedia.org/wiki/Symbolic_link

If you're a Windows user think of a more hardcode version of shortcuts. You can create them in Windows Vista / 7 from the command line through the `mklink` command.

Basically you can set up a symbolic link from inside your Dropbox folder to any other directory and Dropbox will start syncing the folder without you having to actually copy or move files.

Or use a shell extension: http://schinagl.priv.at/nt/hardlinkshellext/hardlinkshellext...