Gambit internals does kind of look like black magic half the time, but the author implemented environments because I asked [0], which is very cool.
The more experimental lisps, the better we can make the future. I love how the lispish community views PLT, and always tries out someone's new idea. (Bayes integration in newLISP is cool. LFE is also cool.)
Xtlang in Extempore is a whole lot of fun [1].
Echolisp has amazing built-ins and runs in the browser [2].
Gambit is great to get C code out of.
I like LFE, but I have to learn more OTP/BEAM to really use it.
Wasp Lisp is a lot of fun for me, and for playing with distributed programs.
They're functions whose args are passed unevaled.
It kills a whole variety of compiler optimizations, but in an interpreter, there's no reason not to have them.