I've downloaded and installed this one (costs 5 Euros): http://autosave.cz/en/about-autosave.html

This one does an incremental autosave (you can set how often you want it to save and how many increments you want it to have before it overwrites the _01 file) and it will save to a second directory at the same time if you want it to.

I had some problems getting it to work and the guy who wrote it was very quick at replying. We traced the problem down to the fact that I have both the 32 bit and 64 bit version of photoshop installed (I use the 32 bit most because it supports the 3rd party filters I use). It will work only with the 64 bit version unless I uninstall it and then it will work with the 32 bit version....I'm a bit loath to uninstall the 64 bit version if I don't have to. I've renamed the 64bit .exe extension to disable it.

My verdict: This works well if you are running only one version of photoshop. Bit of a pain but doable if you are running two versions of photoshop (you will need to disable one as above or uninstall it).