If you like Emacs, and are looking for version that plays well in OS-X land, it looks like Aquamacs is what you want.
As an aside, following the instructions here, to download and install MIT Scheme, will get you ready to self study Structure and Interpretation of Computer Programs. Eli Bendersky blogged his effort to read the book.
If you’re concerned about learning Lisp to use Emacs, you don’t have to. But if you care to dip in, defmacro’s The Nature of Lisp is a good read.
If you’re looking for Python support, check out this write up (M-x run-python just worked out of the box – nice Aquamacs!).
There are many versions of Emacs available for OS-X beyond Aquamacs and the one that Apple bundles. You can find them on the EmacsWiki. The CarbonEmacsPackage is a popular choice, so is Emacs App. I’ll probably end up experimenting a bit with them both.
There is a great set of Emacs tutorials at IBM’s developerWorks.