The Olympus Varimagni Finder is the most versatile accessory I have as it not only provides right angle and 2.5X magnification for my OM bodies but it fits and works correctly for a number of my other bodies such as the Canon EOS, Pentax K, M & LX and various Minoltas.

This finder has a diopter range of −7 to +3 and if properly set, will allow you to achieve much better critical focus then without it. Specs and Instructions for proper use can be found at -> Olympus Varimagni Finder