Wow! I LOVE this thread!!! Suddenly I feel a lot better about my addiction. I'm an admitted 50mm addict, but I have a lot fewer than many of the people here.
LTM (used with an M adapter)
Jupiter-8 50mm f/2 (I got lucky, this is a nice one)
Nikon
50mm f/1.4 AIS
50mm f/1.8 Series E (the two of us have a lot of history)
50mm f/1.8 AF
Lensbaby Composer
35mm f/2 AF (about 50mm on my D300)
Minolta
55mm f/1.4 Rokkor (not quite 50mm, but close enough, this is one amazing lens, sadly I hate the SRT-101 body)
Hasselblad
80mm f/2.8 C
4x5
nothing this size yet (I'm on either side) 🙁
Disturbingly one of the things that convinced me to buy a Leica M6 was the number of different 50mm lenses available. There are several Leica 50's I want, and at least one Voigtlander (I've rented the 50mm f/1.1 and liked it). Having said this, my next Leica lens purchase most likely will not be a 50mm, but rather a 90mm or 135mm. Then I'll start thinking about something like a 50mm f/1.4 Summilux.
As for why, different 50's even on the same camera produce different results.