I mentioned Etch 55 days ago. That's Cisco open sourcing their messaging protocol software.
Last week, while I was on vacation, Google released Protocol Buffer, their data interchange format software.
I just learned yesterday, through Bruce Eckel's rant piece about Java, that Adobe has open sourced something called BlazeDS, their Java remoting and web messaging software.
It'll be interesting to see who else would opt for releasing their own messaging software as open source rather than adopting one that's already open sourced.