Most space conserving mob grinder: One cactus. I get spider loot ALL the time from my cactus outside my door.
In a mob spawner room I got around 4000 Items per hour. I did do something that may be a cheat wich is do it on a skeleton spawner so it made bones AND arrows. so about 4K per hour. No proof but hey just think about it for a second and it makes sense I got about one double chest full every hour (Yes I did sit there collecting items for one hour. Yes it was boring)
I have been able (on my friends' server) to get together over 150 skeletons in one place and have them kill each other. I DID spawn them, but I didn't put godmode on and I lived. I have done that with spiders too.
i have the biggest diamond vein ever (no mod used) 24. 3 cube of 8 sticked together caused by a glitch of chunk. i love them =D and no, i dont have the seed because my computer crashed a week later (maybe jaelous) and i need to reset it =(