I know this has been possible since mobs started to attack animals, but I have never seen it done before. This is a prototype spider separator using chickens.
Note: This was created with HCB off, but I designed it so you could fairly easily fit canals going in.
The inside of the killing mechanism:
The glass in the back houses 5 chickens, which are in range of the spiders by the time they get to the separtion part. All the other mobs fall into the slot to be ground by chopping block, while the spiders go overhead. There is a slot in the roof at the back of the spider killing chamber which douses the wall in water to clear webs (though I don't know if spiders shoot webs without direct line of sight). The spiders usually will not jump the gap, and just walk into the saws, which they press themselves against to get to the chicksens. They don't climb above the saws since the blocks form a lip, trapping them. If they do jump, the water knocks them off the wall.
The clock on the outside that powers the whole thing:
There is a 4-tick pulse that goes around the repeater loop, activating two sets of pistons. One is to the right, which releases the water for the spiders. The other set, which is under the killing mechanism (you can see the line that powers it) opens the chopping blocks briefly to kill other mob types.
I currently have not tested this with a full mob grinder, only throwing in mobs through eggs, though it seems to work flawlessly. If you have any tips on how to improve it, feel free to share.
(Also, the signs with text simply explain what the parts do)
