Such a shame this doesn't work anymore, because Mojang removed this feature. Answer the poll if you think this shouldn't be removed! And I hope sometimes someone from Mojang sees this and put it right back into the game!
I just found a way to create all sorts of things with mob eggs. You must have something to get it, like SinglePlayerCommands, becuase it isn't in your creative inventory.
I couldn't find anyone saying this on the internet, so I thought I'd mention it. *edit* It seems this was once mentioned on the Vechs topic for a little while.
The command in SinglePlayerCommands for getting mob eggs is:
/i 383 [amount] [ID]
In the newest weekly snaphot you can shoot the eggs out of dispensers and it spawns the mob (or something else I will mention later). But you can't use this in just the 1.1 version.
You can use several IDs that aren't really in the game. So I'm not mentioning the IDs for mobs, becuase you can just get them with creative mode. Here they are:
2 : This one spawns experience orbs. If you hook up a dispenser (in the newest snapshot) with a rapid fire pulse. The only thing is, I heard you won't get any exp, it is just the sprite of the exp orb. So this isn't really good for custom maps.
9 : Don't do this. It is a spawn egg for paintings, but it just crashes Minecraft!
10 : This spawns arrows. The arrow will only spawn when placed on a wall, but the arrow will just fall straight down. This isn't really convenient, but maybe you can use it.
11 : This spawns snowballs. It's just like arrows, you can only place them on walls, but then they will drop and dissapear. You can use this for snowing or something.
14 : This spawns enderpearls. The same as arrows and snowballs, but I can't think of a good idea for this.
15 : This spawns an eye of ender. You can place it anywhere, but it will just go to the stronghold and then it disappears or you'll get it back.
20: This spawn TNT. When you place it (or shoot it with a dispenser in the newest snapshot) you will get an explosion. This is just great for custom maps!
40 : This spawns a minecart. You can place it just on the ground, so you don't need rails. This can be really convenient for custom maps.
41 : This spawns a boat. Just like the minecart, you can place it on the ground.
53 : This spawns a giant. This isn't a real mob you can get when playing Minecraft, but they are just huge zombies. These monsters can be good in dungeons of custom maps.
63 : This spawns an enderdragon. I don't really think this is convenient on the overworld, because it just destroys everything. It can be conventient in the end for like a boss battle, but with 5 enderdragons.
97 : This spawns a snow golem. This is just good for custom maps So you don't have to build three blocks. Now you can just spawn one!
200 : This is an endercrystal. This thing is that exploding thing on top of the obisidan towers in the end you have to destroy before owning the enderdragon.
In the next real update (Minecraft 1.2) there will be a bottle o' enchanting (This has already been implemented in one of the snapshots). This will give the player experience orbs when thrown. So the egg with exp. orbs isn't really needed anymore.