I'm not thinking of a digital one, something like this (but obviously not this Olympus model)
http://www.amazon.com/Olympus-AS-ME-.../dp/B000CBSGYE
This "increases the viewfinder performance of the E-300 & E-500 by providing views 1.2 times larger than normal"
I'm sure I've seen ads containing similar things in the back of Digital Photo Monthly. You just clip it on.