K-Meleon used the Gecko rendering engine but with a native chrome and no SpiderMonkey. It also powers Thunderbird and the SeaMonkey browser. I'm sure it turned up elsewhere before it was too 'baked in' to Firefox.
For a short time, there was some attempts (native cocoa and gtk widget) to embed gecko, but it was really not meant to be, it was really hacky and never matured.
Gecko was never designed to be embedded. Really.