Another post questioned how practical it is to compile languages into Javascript. Practicality aside, what interpreters translate from some language X to Javascript and were implemented in Javascript? I'm especially interested in the cases where X wasn't designed with the intention of targeting Javascript. Even if the interpreter isn't totally finished I'd like to hear about it.
You may be curious about the motivation for this question. Javascript certainly can't leverage as much power as assembler, but the flexibility for code distribution and execution may sometimes outweigh that cost.
One use case for these interpreters is a model for teaching and learning programming languages. Consider how useful it is to have a Javascript interpreter in the interactive book Eloquent Javascript. Sure, there may be a performance hit over compiling it natively, but in some situations the cost is certainly worth it. And yes, I realize it's always an option to rely on the server to compile or interpret code, but I'm specifically interested in client-side approaches.
UPDATE: A lot of responses include references to compilers that weren't implemented in Javascript. I just want to stress that the interpreter itself should be implemented in Javascript so that both it and the source language code could run on the browser on your grandparent's computer.
See also this question: Are there other languages than Objective-J that get “compiled” to JavaScript in the browser?