Dropbox is currently one of the most popular file synchronization services on the market. Its client application is available for all major platforms, but up until recently there was no portable version that would allow you to carry it on a removable drive.
For those that need to store the Dropbox folder on a removable drive there is DropboxPortableAHK, a project based on AutoHotkey language that allows you just that. However, portability comes with a downside, and in this case it means using a lower version of Dropbox (1.1.45).
The reason for this is that newer versions of the service encrypt the configuration files and the portable build cannot customize the path of the Dropbox folder.
If you are fine with this, you can use DropboxPortableAHK with multiple accounts on the same computer. Also, in order to protect the synchronization folder, it will automatically kill the Dropbox process the moment it becomes inaccessible (e.g. when you remove the USB drive).