Windows: You can do a lot more with the free cross-platform utility Dropbox than sync files. In fact, with the free command line tool Akira, you can administer, and grab non-Dropbox files from, any computer linked to your Dropbox account.
Akira is a bit of a geek tool that requires two Windows computers — one running a server process (Latte) and another issuing commands through a command line client (Akira). That's not all that new, but the Akira tool sets up the connection through a single, stand-alone folder inside your Dropbox folder, and doesn't require installation. With Akira's text commands, you can do a lot of really neat stuff. You can list, navigate to, and grab files from anywhere on the remote computer, grab a screenshot of what's going on and save it in your Dropbox folder, and, perhaps most helpfully, start and kill remote processes and run applications, or even launch a web site in the default browser.
Akira's Readme offers a number of ways to set up the Latte/Akira combo, with access to other people's computers through Dropbox folder sharing, and jumping onto your remotely-controlled computer from anywhere with the previously mentioned app DropboxPortable.
I tested out the Akira/Latte remote control between Windows 7 systems linked to the same Dropbox account, and while there was a notable bit of lag — something to be expected when waiting for a file change to be noticed and synced — it worked as advertised. You'd likely want to set up Latte as an auto-starting, Administer-level process on a system you always keep on, and make sure you've got a really good password on that Dropbox account if you were interested in using this. As a convenient remote access trick for those not quite up on setting up a virtual private network with Hamachi or remote control with VNC, however, Akira might just be the ticket.
Akira is a free, no-install-needed application for Windows systems with a Dropbox account. Tell us how Akira could be useful for your own systems, and offer up usage advice, in the comments.