In automotive circles, the "sleeper" has a long and roguish history. Take a plain-Jane car and throw some serious performance bits under the hood and prey upon the unsuspecting (bwahahaha). A frumpy four-door with a quiet (at least at idle) exhaust makes it even tastier :-). When it comes to old SLRs, there is no such post-purchase hopping-up per se, but there were enough models that followed the spirit of the sleeper as far as looks and features went to make things interesting. The bonus today is that you can snag one of these soporific snappers for a fair bit less than their more-celebrated contemporaries, while giving up very little (if any) outright performance. Now, if I happen to leave out your favorite flies-under-the-radar film-burner, don't get uptight. Feel free to mention my misses in the comments, and who knows, maybe we will have to do a sequel. So...in no particular order...
Suffers from a quarter-century and counting film and manual focus SLR addiction. Has recently expanded into 1980's AF point and shoots, and (gack!) '90s SLRs. He even mixes in some digital. Definitely a sick man.