Learn Scheme - A Complete Environment for Learning Lisp
Up until now, in order to tinker with Lisp, you'd need to download some heavyweight Lisp environment (hope your platform is supported!), configure language options, and generally spend lots of time not actually learning anything.
Now, you don't have to. Learn Scheme is a complete Lisp-learning environment that fits comfortably in your browser. Thanks, Web 2.0!
Additionally, through the magic (i.e. liberal licensing terms) of the Creative Commons, Learn Scheme also includes a classic textbook on programming in Lisp: Structure and Interpretation of Computer Programs by Abelson, Sussman, and Sussman (originally published by MIT Press).
Scheme is a minimalist dialect of Lisp. It boils Lisp down to its clean, simple essence. Structure and Interpretation of Computer Programs similarly boils programming down to its essence... which it then builds up into a full-blown Lisp interpreter and compiler. That's right, you get to learn Lisp by implementing Lisp in Lisp! That's a lot of Lisp!
If you're intrigued, continue on to get started with the wonderful world of Lisp! If not, go read some of Paul Graham's essays on Lisp. Still not feeling motivated? Here is why you should be inspired to tackle Lisp:
"LISP is worth learning for a different reason -- the profound enlightenment experience you will have when you finally get it. That experience will make you a better programmer for the rest of your days, even if you never actually use LISP itself a lot." -- Eric S. Raymond, How to Become a Hacker