I've just released my application to the public, and I'm working on an updated version, but I really think I should keep track of ALL the code changes. In case some functionality suddenly starts failing, with a history of all the changes I made it would be a lot easier to figure out where I messed it up, in case the problem wasn't already there.

The ideal would be to have a super fast computer with a huge hard drive and an application that automatically saves a backup of the whole project every time I change a line in the code, with some file comparison tool that would show me every difference between any two backed up projects, but that's not really possible for now.

So, do you know any application that makes it easy for a programmer to keep track of the changes made to the source code?