Five unidentified different species here.
One of them doesn't look much like a cactus or succulent, but it was sold by a vendor who almost exclusively sold cacti or succulents, so I put it here.

#1 Opuntia microdasys?
#3 Dorstenia, perhaps foetida

#1 Opuntia
#2 maybe Parodia (if its gold instead of purple)
#3 Dorstenia
#4 maybe Copiapoa
#5 maybe crested form of Mammillaria elongata

Thank you all!
Daisyl, #2 is greenish yellow. Yellower on top and greener on the bottom. Discoloration due to rookie mistake or is that simply how it should look? Here's a picture.

Then yes, Parodia.

Last one looks like leninghausii
The spines on mine become paler towards the bottom of the columns, if that's what you mean about discoloration?
Yellow Tower (Parodia lenninghausii)

The crested one looks like a leninghausii indeed. Since I can't post links yet, I pulled this picture of a crested leninghausii off the web:



I was worried that the yellowing is due to some mistake I made. I have two of the species pictured in the fourth photo. They both were completely green at first. Now one is green and the other isn't. The same goes with the other Parodia, which now has a nice gold-ish hue on top.

The one with the four arms lookes like a leninghuasii, but I wasn't able to find a picture of that species with arms online. Hmmm...

Could be leninghausii! It may have been taken from a large clump! I think they branch with age, check the pic of leninghausii near the bottom of this page.
http://lepetitpoucet.free.fr/S...

I think sometimes damage to the top of a cactus can cause it to "branch." No idea what made a Mammillaria my Mom has form two heads, but it's now two headed, lol.

I thought it could have been damge too hamwild, or the soil level was once level with the arms! I find quite often too much soil applied above the original soil level burying part of the plant and when removed exposes an older cacti with an undesirable lower stem. Maybe not the case here tho.

I think you guys are right! Here's a closeup of the top, if it helps.

Yes hunter parodia leninghausii

I think number 4 might be a Matucana madisoniorum. Can someone verify? Thanks!

With age, some single stemmed Mammillaria start to divide dichotomously. Eventually, it will look like a whole colony with a common trunk.

#4 I thought about Matucana madisoniorum but to me, the globs are very distinct on your's and the big spines are weird. Mine are almost smooth and spineless. You will know for sure when it blooms - the flowers are bright orange.

Mystery solved! The Dorstenia is a Dorstenia crispa.