One of my favorite lenses and maybe even the cheapest lens I own, is the Fujian 35mm F1.7 c-mount lens. A tiny lens, mostly used for CCTV cameras. These lenses can be bought in China for around 20 dollars, often including the adapter and shipping. I bought mine after seeing a raving review of Nate Burr (Blunty) on Youtube, a couple of years ago. Since then I have shot quite a lot of pictures with it, including some of my favorites, and I (still) like a lot. Hereunder you'll see some of these pictures, including 7 reasons why this is one of my favorite lenses.
The Fujian 35mm F1.7 c-mount lens.
The rendering of the lens gives pictures a real vintage look right out of camera.
At F4.0, the lens still gives a nice background blur but sharpens up a lot, isolating the subject in the foreground very nicely.
Point it towards some scattered light, caused by water or light coming through some leaves and you're in bokeh heaven.
Talking about bokeh. Ofcourse the swirlyness of the bokeh is what the lens is famous for.
The combination of sharp centre and extremely blurred corners gives some pictures an almost tilt shift look.
Let the light come in from the side and it flares like crazy. Just how I like it ;-)
Focussing by hand is fun and with some practice, and a couple of tries, even a fast moving subject can be captured.
Link to the Blunty Youtube Video: Cheap-Ass Bokehlicious Lens - Best lens you'll get for $20
This is the website were I bought mine: www.DinoDirect.com