Industar 50-2 50mm F3.5
I bought this partly because it was cheap (£12) but mostly because it was very small. The inner ring adjusts the aperture and it is rather fiddly but otherwise it is a fun lens to use. Produces nice colour and contrast and is sharp enough.
Shame the NEX to M42 adapter is bigger than the lens, which somewhat negates the benefits of such a small lens.