First one is a form of cereus and second a form of echinocereus! Not sure what type exactly though but its a start :)

the last one might be some type of pilosocereus.

I have one that is like the third photo, but I have no clue what it is! I'll be interested to find out.

Thank you, everybody, this helps a lot. Anyone else with more details?

the third may be a type of rebutia, but i'm just guessing.

#1 is Marginatocereus marginatus aka Mexican Fence Post.
#3 is a Mammillaria
#4 has a severe fungal problem and should be removed from the cacti!
Jordi

Oh, wow! Well, thanks again, everone.