One of the staples of any efficient SkyBlock island is a mob spawner. I use a design that I saw on one of CaptainSparklez' SkyBlock videos and decided to make a tutorial for you guys! With pictures!
The basic mechanic of this spawner works with natural mob spawning in a dark area. As the mobs spawn, some of them will fall into the water streams and be pushed into the hole in the middle and fall to their deaths. I made it a 30 block drop to make sure they die.
Pros of this design:
No redstone required at all!
Works all day and night!
Relatively easy to make*
Requires a TON of cobble, at least 14 stacks. More if you have to pillar your way up to it repeatedly.
Dangerous to make* - If you don't light up your area as you build the spawner, you will get blown up or shot off. I learned that the hard way ;-;
You will not get spider eyes from this grinder unless you damage the spider before it dies. I usually build up a supply of arrows, then just shoot the spider as they fall.
Step 1: Water trenches
Pillar up 30 blocks from where you want the mobs to land/die. From there, you'll build an 8x2 platform with sides.
After that, you'll make a cross pattern with the trenches for a total of 4 trenches.
At the far ends of the trenches, you will need to put two water source blocks. This will push the mobs who fall in towards the center.
Step 2: Spawning platforms
After you have built the trenches, connect the sides of them with cobble, making an 8x8 platform
Now make another layer. Technically, the first layer isn't really needed, but it just makes it easier to build on and you have that many more blocks to increase your island level.
Step 3: Sides and Roof
This spawner isn't going to allow Endermen to spawn in it and mess things up. Make a border around the platforms two blocks tall.
Now cover the spawner and spam torches on top. It's unlikely that mobs spawning on top will give you any trouble, but it's better that they spawn inside the trap so you can get the goodies
Step 4: Profit
If you followed the steps, you should have a functional spawner that works day and night. One of the latest mechanics is that mobs will avoid long falls if they can, but the water is stronger, so it will push them regardless.
Be careful. I didn't die once while building this spawner this time around, but I came close several times. YOU MUST LIGHT UP YOUR AREA AS YOU BUILD OR YOU WILL DIE.
It is much easier to make this spawner as a SkyLord as Skylords have access to /top, which will negate the need to pillar up each trip. It is possible to make this spawner as a user or Islander, but it's more challenging that way.
There are probably more efficient spawners out there, but this one is efficient enough to get you a steady supply of mob drops.
If you have any questions, please let me know!