Lambda the Ultimate

Polyglot is a compiler front end framework for building Java language extensions that doesn't seem to have been mentioned here.

Amongst the extensions is an implementation of nested inheritance which, I admit, I don't completely get. There's a discussion (moderately critical) of the paper in the context of OCaml that starts here.

This caught my eye while scanning the latest Caml Weekly News - a useful summary of the (rather high volume) (O)Caml list.