You can take Leah into Whimseyshire with you- she does around 800 damage a hit in inferno mode, but can sometimes block mobs and w/ not.
To take her w/ you, start a game on the quest "the broken blade" -- and just walk in.
Edit: Oh -- and she's invulnerable. So when you die she will still be shooting away.
Edit 2: This is useful because she will stop the mobs from camping your body- nice for when you're just hunting treasure chests. (clouds)