http://hackaday.com/2014/09/18/diy-magnetic-stirrer-looks-profesional/
http://www.nationalhomebrewclub.com/wordpress/diy-stirplate/
Only tip is if your an electrical eejit like me get something like this to make sure the stir bar doesn't keep flying off. I got mine in Maplins. It works a dream.
http://ie.rs-online.com/web/p/products/4179728/?cm_mmc=IE-PPC-0914-_-google-_-3_MPN_Exact-_-Como+Drills%7CPSF_421230%7CDC+Motor+Controllers_919D2P_E
and if you know what you're doing (thanks to Eoghan and Brian) apparently this is how you do it
http://www.electronics-tutorials.ws/blog/pulse-width-modulation.html