There is also the 'Rolling-ring' drive that is interesting and has been around for a while, I think it is used mainly in lower load applications, I have come across them in CNC back gauges for shears & brakes etc , Amacoil is one manuf.
http://www.amacoil.com/ due to the possibillity of slip under stall or high load, a linear scale is usually used to detect actual position.
Al