Two easy introductions to monad transformers in Haskell.

The first is a PDF, Monad Transformers Step by Step, that builds up complex functionality step by step using monad transformers.  It shows in detail how to combine transformers, and why you would want to, two things which had thoroughly confused me.

The second is another brief post from A Neighborhood of Infinity, Grok Haskell Monad Transformers.  This shows how to work with a stack of monad transformers using lift.