I'm surprised no one has mentioned the Truecut rotary cutter...
http://www.graceframe.com/site/rotarycutting. It has a guide for the cutter to ride on which makes it much safer to use. I think this will be my next one to buy. My sister has one and likes it. If anyone has used this one I would love to hear from you. (you do have to buy the ruler). Sandi