My primary world, home of the original Glass Island and Skyrail system.
My modded world, featuring the delightful 
Super Hostile - 
I have a bit of a spider problem.
One of the first outdoor things I built, after tunneling around inside the island like a termite, replacing the surface bit by bit with glass, was a mob trap - a drowning trap, to be specific.
It does a great job on Creepers, Zombies and Skeletons.
Bye Bye, Mr. Creeper
However, pigs regularly flow right under the one block gap and splash around happily in the collection area until I manage to smack them down with a sword. Today I realized another mob is capable of slipping through.
Finally I just headed down myself, and the newest fuzzball respawned nearby, happily bouncing along. He joins the rest of my pack... I'm not a hoarder, honest >_>
I do think I'll need a bigger kennel soon.
Back to the spiders. While I do have some cactus rigged on top of my mob trap and horizontal crenelations to keep spiders from assaulting me inside the trap, it's not very good at actually killing them. Spiders are also more problematical than most mobs, with their unnatural ability to spot a tasty player morsel right through solid stone and dirt and then proceed to skitter and breakdance over their heads all night long. Wouldn't it be nice if I could walk that smug spider right into a deathtrap? Thus the Spider-cinerator was conceived.
The topping half blocks keep anything over one block tall from blundering into a fiery death by accident (namely, me. And yes, I did test it out. It's built next to the water trap for a reason. ) The original version was actually only a 2x2 pit of lava, but after attracting a spider and dancing under the lava pit like a fool, I have concluded that it's very fiddley to line up a 2x2 mob with a 2x2 hole.
However, I did get an awesome shot of my first toasty test subject.
Only time will tell if the 2x3 pit is any better than the 2x2. I seem to be fresh out of spiders.
