What is the best Scheme or LISP implementation for OS X?

I am looking for a version of Scheme or even LISP that I can use to recover some lost Lisp development skills. Some web capabilities would be nice but not essential.

I’ve looked at Plt and MIT scheme and, while both look pretty good, the Plt seems to be more feature rich. I’ve also looked at Lisp implementations but all of the seem quite expensive.

I favor free/inexpensive implementations as this is truly likely to just be occasional hobby programming. What recommendations would you have?

How to enable the Research Assistant in Xcode?

I created an empty project in Xcode and have added all of my source code files to it.
I do not have any build options/configurations set since the project was empty and I use my old Makefile from the command-line to build the app. Everything works – but…

My problem is that it is a tad annoying not to have the information at my fingertips since the Research Assistant isn’t showing any information at all!

The Research Assistant is shown but it doesn’t show any information, I guess that it must be instructed to so somehow – but how? Is there a way to enable the Research Assistant and how?? Also, note that I have all the documentation available (but not from the Research Assistant)!

Thanks in advance!
/John