I have an old small (about 15'x20') post and beam type barn that is quite a bit out of square (12-18"). I need to try and straighten it in order to make it's usage safe.

Any ideas? I had thought to try pulling it square by using a come-along or two. I would drive a long post into the ground at an angle, attach the come-along to the post in the ground. Then attach the other end of the come-along to the top of the barn beam/post and gradually pull it straight. When/if it gets straight, block it with some cross type bracing. The barn has three posts, and I had also thought to use a towing strap to connect the other two the post being pulled straight.

Does that sound feasible or just plain crazy?

It sounds feasable. One thing is that I would consider is using some cross bracing once the barn is straight. Its nice to see someone trying to preserve their barn. around where i am most people just tear them down and start over. Best of luck!