TinyScheme
Since 2000
TinyScheme is a small Scheme interpreter I developed out of an even smaller one called Minischeme. At the time, I was working at a company called Ovrimos, which was developing an SQL server. Among other things, I developed a small HTTP server to serve its Web UI, and looked around for a solution to use for server-side scripting. MiniScheme came very close to what I was looking for, so I took it and extended it. I soon quit that company, but interest in the (now very rarely updated) project persisted.
You can visit the TinyScheme site to get an idea.