Windows: There's a trick that syncs any file or folder to your Dropbox account, though it involves terminal commands that, if poorly executed, can cause some headaches. A small Windows app does the same syncing, with a right-click or manual folder picking.
After installing the small app, you can run it, tell it where your main Dropbox folder is (if necessary), then hit a button and do the standard Windows folder-picking procedure to sync any folder on your hard drive to your Dropbox account, through the creation of a symlink. Even easier, though, is right-clicking on any folder and picking "Sync with Dropbox" from the context menu.
I've used symlinks before to create backups of friends' messy desktop files and important documents, and it's been great to get the emails about how Dropbox saved their butts in an oh-shoot-where's-that-file situation. If you've got another good use of Dropbox sync, you can get evangelical in the comments. Dropbox Folder Sync is a free download for Windows systems only.
Dropbox Folder Sync [Satyadeep K via Ghacks]