I got asked to write more detailed about old lenses you can adapt, but I'll write here because someone else would benefit as well, I think.
First - you can adapt following mounts to EF:
Second - You can find very god and very bad lenses on those mounts, so you should do research before purchase and always check the lens for problems (lose focus thread, stuck aperture, fungus on glass, damaged etc)
Third - Especially with soviet lenses the quality varies, so you can have two lenses with same designation and one is stellar, other is bad.
Fourth - Lens worth getting:
- Helios 44[Y]-[X] 58mm f/2.0 – [Y] means mount – where M stands for m42, K for Pentax K, the [X] at the end is sharpness, 1 trough 7, higher the number – sharper the lens. It based on Carl Zeiss Biotar 2/58. One of, if not the most popular soviet old m42 lenses because sold as kit lenses for Zenit cameras.
- Jupiter 37A 135mm f/3.5 – there is also multi-coted version, based on Carl Zeiss Sonnar design, it is believed to be successor of Jupiter-11 witch is a lens with similar parameters and characteristics, but with m39 mount. Razor sharp lens, when stepped down, one of my favorites.
- Industar 50-2 50mm f/3.5 – one of the most common old m42 lenses, is also common in rangefinder cameras and older m39 mount, grate for macro photography.
- Mir-20 20mm f/3.5 – wide angle lens, based upon Carl Zeiss Flektogon 2,8/20, no the sharpest lens in corners, flares quite easily. Outperforms most of the kit lenses for DSLR’s tough. Rear filters – so rather hard to obtain, was supplied with 3 filters – uv, yellow, orange. There is more modern version M and also multi-coated
- Mir -1 37mm f/2.8 – Highly praised but not really m42 lens, it is m39 mount, that you can adapt with simple ring, might not focus to infinity without adjustment, because m39 is 44.2mm not 44.5mm to film plane. This lens received “Grand prix Brussels 1958”. It is bit wider than normal lens, based on Carl Zeiss Flektagron, It has new sibling Mir-1b witch are true old m42 lenses. Flares really easily, but does that really beautifully.
- Industar 61L/Z – 50mm f/2.8 – a nice normal lens, design based upon Carl Zeiss Tessar, with rather exotic Lanthanum glass witch is thought to be a little radiative, but also with good optical characteristics. Other is rather peculiar iris design, witch halfway mes nice star sharped bokeh. One of my favorites for HD DSLR filming.
Also worth to mention:
- Jupiter 9 85mm f/2.8
- Helios 40-2 50mm f/1.5
Also you can find many Zeiss m42 lenes and most of them are grate tiough I would say they are overpriced. There also are Praktica lenses that are mostly really good especially their 50mm one, that is sharper than Canon's 50mm f/1.8
there are many good Nikon lenses, but they might feel odd because they turn the other way for focus, and I'm not so familiar with them. I've mentioned lenses that I personally can vouch for, so if someone has something to add - please do!