Certainly a Haworthia, looks like H. cooperi.

Mammillaria elongata:) Could be spider mite damage, but I have a feeling it's the soil. I think if you switched to a much grittier mix the cactus would be much happier. A good start would be 60% perlite or pumice. Depending on where you live pumice won't be easy to find. If you live in California you should be able to find a little bag at OSH. I like pumice, but perlite will do the job. Then 40% cactus mix. If you end up using perlite the mix will be very light. This is good. Your mix should fall apart when dry. It may be a good idea to get a terra cotta pot. Terra cotta allows moisture to evaporate. I'm sure you know this, but there should always be drainage holes:). Switch out the mix and keep your eye on it. If you notice the brown is spreading more and more you may have mites. One last thing when repotting make sure you repot bare root. Clean all that old mix off the roots:). Wait a week to water after repotting. Goodluck:).

Its a pilosocereus, with a few species in mind.

I think it's a (mainly) Brazilian cactus, Opuntia (some say it's a Consolea) species for sure. I don't think it's particularly etiolated, Rob, if at all - that's the way Consoleas grow. Flat-Yet-Phat Cactus Species