There's a tool in GIMP (another free image editor) called the perspective correction tool which lets you select part of the image and drag all the corners independently of the others. This lets you distort the selection as though it were at an angle or "restretch" the image by eye until all the edges are square and it looks as though you are head on.
Aside: If you think that sort of thing is cool check out Getting into the picture (
http://plus.maths.org/issue23/features/criminisi/index.html)
Once painters started using perspective they were actually encoding distances from their eye into their works. The result is that the layout of the scenes can be partially reconstructucted and there are a few movies on that page (1/4 down and 3/4 down the page) where they fly through famous paintings.