Can someone please explain the player head plugin. I would like to know what mobs drop heads, and how to obtain them.

All mob heads are very rare. A looting III sword raises your chances somewhat. There is a 1-3 chance if you kill someone you may get their head. If you pay 5000 you can own your head via a show at adminshop. You can also paysome 5000 credits to buy their head and give it to you.

so even mobs like bats and enderdragon?

bats yes as i have a bat head on classic. But i don't think the dragon is possible.

I saw this image on the minecraft wiki. Is this all the mob heads we can get? and whats with the block heads? How could i get the steve and herobrine heads?

So far, I've only seen the heads that are on the top row with the exception of the chicken. I think that I've seen the cave spider though I'm not sure.

The blocks are probably able to be spawned in but not obtained naturally. I would have so many fence heads and log ones if they could be obtained by looting.

The article it was taken from is this

http://minecraft.gamepedia.com/Mob_head

It appears that most of them are spawned it using /give.

if they are spawned in with give, how would i do this on single player?

found the bukkit plugin site for player heads here are the default drop rates and all the possible mob heads available.

That article has all the commands ect