What are some good online/free resources (tutorials, guides, exercises, and the like) for learning Lambda Calculus?
Specifically, I am interested in the following areas:
- Untyped lambda calculus
- Simply-typed lambda calculus
- Other typed lambda calculi
- Church's Theory of Types (I'm not sure where this fits in).
(As I understand, this should provide a solid basis for the understanding of type theory.)
Any advice and suggestions would be appreciated.

It might be nice to work through Structure and Interpretation of Computer Programs, which is available online for free. This book is an introduction to computer science and the programming language Scheme, which is a flavor of the programming language Lisp, which is based on the lambda calculus. Although it is not strictly a book about the lambda calculus, it might be fun or useful to gain some hands-on and "practical" experience with the lambda calculus by reading some of this book and working through some of its exercises.