A short video showing the canon 50mm 1.8 attached to an Opteka .35X wide angle adapter, which sells for about $30. The converted focal length is 17.5mm (closer to 20mm in practice - on a crop sensor body, that makes it right around 35mm) Surprisingly, the adapter barely affects exposure, maybe a decrease of 1/3 of a stop, which can be easily compensated by increasing ISO. Another benefit is that the adapter converts the lens to a macro with a minimum focus distance of ~3 inches! No wide angle lens I've ever used focuses that close. You do lose some image sharpness, but for video purposes, I don't think it's a big deal. It could actually add to the aesthetic in some cases i believe.
I know that Sigma makes a 20mm 1.8, but that lens is nothing to write home about and will set you back $500. It won't look like the 24L prime but then again, it's only 30 bucks :)
f2.2, shutter at 50, ISO 200, 1080p 24p, T2i, footage uploaded straight from the camera.