I've been using Scheme a lot lately, and when looking for information on the language I discovered this wiki. After a while, I noticed that the wiki has been overrun by spam bots -- so I started reverting the changes they made, banning their links, and cleaning things up a bit. However, getting revert permissions would be more helpful because I currently have to delete every line of spam manually. At least there are diffs to help restore old versions, though. :)
I'm most familiar with implementations that compile through C code. Gambit and Chicken are my favorites. A lot of the stuff I do is related to multimedia or games. I'll write part of the core in C or C++ and build on top of it with Scheme. This gives me all kinds of benefits like the REPL, loading scripts (and compiled files, too) but I still get good performance -- and all in a much nicer language. This also lets me use Scheme for all kinds of projects since I'm technically just creating portable C/C++ code. :)
I also use Common Lisp to a lesser extent (SBCL and CLISP for that) -- mostly just playing around though.