Are there any open source Lisp compilers suitable for real-time embedded applications? I.e. with incremental garbage collection, customisable memory handling, small footprint, etc.
Edit:
To clarify, by "compiler" I meant native code, not bytecode interpreter (though the suggested interpreting implementations for microcontrollers are interesting for being a lot smaller than what I thought possible!).
Take a look at Picobit and the code, which is a Scheme for microcontrollers. There is also tinyscheme, but I don't know how suitable it is for embedding into smaller processors.
There are some implementations that compile to C code, e.g. ECL (Embeddable Common Lisp). There is also a very recent library that compiles a subset of Common Lisp to a subset of C++ for the Arduino.
You mentioned "real-time" so nonchalantly, but getting real-time with garbage collection is not trivial. There do exist real-time garbage collectors, but I am not aware of any Lisp implementations using them yet.
There is also Hedgehog Lisp from Oliotalo, licensed under LGPLv2.1, with standard library licensed under BSD.