Guile VMThe GNU project's Guile has some very interesting approaches for a concrete VM. Its VM is the Scheme programming language, which is powerful enough to have arbitrary frontends to other languages. So you can write software in any language and with any syntax (a compact byte-code syntax is feasible), yet run it inside a Guile environment. This might be any program linked to the Guile library. Hence, any program can be extended using any language.
Although the concept is quite nice, the implementation looks unhappily quite bloated and inefficient; as a result it's acceptable as a glue "scripting" language, but not fit for efficient computation.