Top right: Senecio crassissimus Humbert
Bottom right: looks like Echeveria'Perle Von Nurnberg'
Top left: Haworthia fasciata (I think)
Bottom left: looks like a Pachyphitum (don't know which)

I'll confirm that the top left is Haworthia fasciata.

For the bottom left, I would guess an Adromischus of some kind.

I'll confirm that bottom right looks a lot like an Echeveria, though I think it may be 'Moonglow.' (Though -- cultivar names aren't always very stable / reliable, so it could well be one, the other, both, or neither.)

Not sure the Semp will be happy with the large rocks around...they like a more "bare" soil...

Hi all!!

It is a very nice collection but I think that the top left is a Haworthia attenuata. I have it in my collection too!
Check this url, I believe it looks quite similar to yours(and mine):
http://www.cactus-art.biz/schede/HAWORTHIA/Haworthia_attenuata/Haworthia_attenuata/Haworthia_attenuata.htm

Regards
Christos.

You guys are awesome! Thanks for all your help!

Cheers