All depends what sort of standard you want. If you're doing it just for your own racquets then the one I'm about to suggest is probably unnecessarily pricey. I went through a few when I worked stringing racquets for a bit, and found the best one was the Babolat sensor stringing machine. Very easy to use and the machine does most of the work. The one at the top at the following link, I think you'd be able to get it significantly cheaper than that price though.