Software to aid using camera as a scanner

I want to digitize some index cards with my camera. I'm looking for a program that would automatically fix geometry on the shots (as you would expect the cards come tilted on the picture). I have an app (droid scan lite) on my android phone that does exactly that, but I would prefer to do it on my pc (the phone camera has poor quality and it's slow and focuses badly while I have a decent slr). If the program is open source it's an advantage, cross platform -- even more so.

Question by xxzoid on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.