The Erlang system is a general purpose network based agent architecture, with strong support for TCP/IP based messaging and telecomunications applications.
The Capriccio project at BSD uses low level system operations to switch multiple stacks, and hooks core blocking operations such as socket I/O in an attempt to allow cooperative multi-tasking of ordinary C code.
It also used advanced analysis based on CIL and CCured to assist in optimisation.
The flxcc tool provided by Felix is also based on CIL
The IO language also provides lightweight threading by C stack manipulation.
The Lambda The Ultimate weblogs are always great for discussions about languages.
Probably the best database of online academic papers on CS around is the NEC/IST citeseer system. Truly awesome!