One thing that I find invaluable from time to time is the ability to get back an old version of a file that has changed many times.
Now a lot of the Sync tools (if you use them to backup your files) out there will give me back the latest version only. The only app i've come across that is portable, requires no centralized repository and "automatically saves a backup of the file" (actually i define a directory and every file saved there gets versioned to another directory i've defined) as well as manage how many versions to keep etc is File Hamster (http://www.mogware.com/FileHamster/) but this is not open source and contacts home a bit too much for my liking, but i've put up with it because it's the only thing i've found to do the job I need.
Does anyone know of any other open source app out there that would do a similar job?
You can try this
I just tried it (it has an installer) does this mean it's not portable?
Bazaar will run on any system that has python installed. It is possible, though not friendly, to install portable python on a flash drive, include the dependencies of Bazaar, and achieve complete portability in that manner.
This is where I heard about the second option:
I've not yet been able to test that solution, so I cannot comment on its veracity myself, but the OP of that link did respond that he/she utilized it.