How to make a sound loop?

If I want a sound to loop perfectly, how can I do that? Can any of the free/cheap audio editing tools help me out with this.

Basically I just want to edit an ambient sound so that it can loop continually. It doesn't matter if it's repetitive, I'll be layering some other sounds over it so it'll never notice, but it will notice if it glitches where I loop it.

I don't know what it's like but you could take a look at Audacity:
http://audacity.sourceforge.net/

Well, assuming that the sound is not too different sounding at the beginning and end, you just have to zoom in on the sound and clip it at the beginning and end where the sound crosses the 0 line. Also if the beginning starts at 0 and goes up, then you want the end to start below 0 and go up to 0, so that you have a continuous wave.

Thanks guys. I already have Audacity, just wasn't sure what I was aiming for. Sounds simple enough. Just clip it as though it was a tiling texture. So that the line matches and is heading in approximately the same direction. Thanks, I'll give it a go.