Bought a 10s Deore 610 (newest model) rear mech to dismantle and convert to a chain tensioner for my singlespeed. A very quick test fit showed shimano have changed the geometry of their mechs so they no longer do the job. Has a small (10x3mm) patch of the anodising rubbed off on the arm the cable attaches to from trying to get it set up for singlespeed. Can't see it when fitted.
Never been ridden, been fitted to a (then) unridden bike for ~10 minutes. Still has packing grease on it. Good to stick on a bike with rear shifting that's just not as crisp as it used to be!
Like this: http://www.chainreactioncycles.com/shimano-deore-m610-10-speed-rear-mech/rp-prod108792
Long cage, black. Cheapest price I can find it is £30 on bike discount.de once you add on postage so lets go £25.
Mat