There are a couple of ways to kill monsters if you want it completely automatic;
* Dropped to their death - as mentioned above
* Lava "roof" 1 block above a water stream - will push mobs into the lava in head height and kill them (although there is a small risk that the drop gets incinerated)
* Cactuses - same as for lava, you push mobs into them with water, here as well the drop might get destroyed if unlucky.
* Two layers of water - one layer is in head height, resulting in the mob eventually drowning (they can't swim up in it due to the layer of air in between). The second level of water will still take the drop away safely. No risk of losing the drop.
* Crushing pistons - will essentially suffocate mobs as well
The four last options allow for easier placement as you don't need to worry about the required fall height, you can essentially build the spawner at any height. But they are on the other hand more advanced to build and get working correctly.
As per collecting the drops, doesn't the newest version of minecraft contain hoppers? But maybe that wasn't implemented in the current server version?
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Last edited by DaedalusAlpha; 4th February 2013 at 11:36 PM.