libwasm
libwasm is a cross platform Web Assembly JIT engine with no external dependencies and good performance. It was created to allow non-web programs to easily support web assembly. In addition to providing an interpreter libwasm also specifies a stable ABI and libc allowing cross platform execution outside of the web.
WasmRT
WasmRT - The web assembly runtime builds upon libwasm and is intended to allow applications to ship in webassembly without requiring recompilation on different hosts. WasmRT provides a consistent and cross platform ABI intended to work well with command line applications.
Why
Many programs embed language runtimes for extensibility such as Lua or Javascript. While great for most purposes traditional embedded programmability forces a specific language on the user. Wasm allows extensibility to be added in a language neutral way.