Singleplayer commands lets you use multi-player commands in singleplayer.

it is on the minecraftforums.

1 command includes /spawn which lets you spawn all existing mobs *like the giant that doesnt spawn naturally so you need mods.*

You can spawn any mob that exists in game even ones that were unused that dont even spawn *Like they are not un-coded.*

But there are TONS of other commands besides /spawn.