A while ago there was a plugin on a server I played on which allowed you to get spawn eggs for a mob if you simply threw an egg at it. There's a HUGE hassle to moving mobs across long distances and I think it'd be nice to be able to do that and even sell some mobs. After a very short google search, I came up with EggCatcher - Bukkit
Just installed this. I like how it respects the GriefPrevention protections. Give it a test and let me know