The eye is an great example. Not only is the mammalian eye horribly designed (as the article Thor 2 linked too points out all the, for lack of a better term "wiring" for the eye is run along the actual surface of the eye which leads to a really large blindspot in both eyes where the hole where all the wiring leaves the eye goes through to the point that a lot of visual information is filled in in what amounts to post-processing.) but... other types of eyes evolved that don't have that. Squid eyes evolved differently not having that weird wiring problem and having a completely different (and arguably superior) mechanical method of focusing.
