Today's 9/10 speed set ups are reliant upon fastidious alignment if they are to work properly - usually this isn't an issue but in the event of a spill, the rear hanger is often the first casualty.
This Cyclus alignment tool works using the real wheel as a reference point. Remove the rear mech, apply a bit of grease to the threaded end of the tool and offer it up to the hanger. Tighten snugly (this is best done with the bike in a work/race stand or even placed on a large workbench on its side). Starting from a 9 o'clock position, rotate the tool, keeping a watchful eye on the guide pin (positioned at the other end of the tool) in proximity to the wheel, where it deviates from a consistent line is the point at which to straighten.
This took me a few attempts, with gentle pulling movements and a softly, softly approach but a bent steel hanger returned beautifully. The construction is solid, comfortable and pleasant to use and at a shade under £22 it will owe you nothing even if you only use more than once.
It's designed primarily for use on steel frames, but might also prove effective on elderly/budget alloy frames without replaceable hangers.