Playing an instrument you already own. This is my favorite hobby, as well as the cheapest. I just need to stop buying books of sheet music and CDs.

Photography is like playing my violin. I own a digital camera and a friend gave me some software, so now the fun is free.

Surfing the Internet and talking to friends on Mudcat, if you own or have access to a computer is great, too.

Some kinds of exercise, like yoga and walking are free. Of course, yoga lessons cost money.

Reading all the books you've already bought.

Cooking. You have to eat anyway.

I'm dealing with chronic unemployment, so I must indulge in cheap or cost free thrills. (I can anticipate some awful responses to this. Please don't!)