Originally Posted by Miguel 
Surely the Mamiya/Sekor M42 preset 60mm f 2.8 macro lens should be on your list. Fun and excellent.
M
I've actually got this one (but labeled Yashinon/Tomioka) and it is indeed very fun. Picked it up at an estate sale but haven't used it much yet.
Some examples (first shots I took with it, actually, and still the only):
