JINI/Apache River: http://www.jini.org
Newton: http://newton.codecauldron.org/
Could be a drop in osgi component to add JINI capabilities with little work!
JXTA: http://www.jxta.org/
Somewhat compares JXTA, JINI, and GT4: http://wiki.perfsonar.net/jra1-wiki/index.php/Technology_evaluation#Globus_Toolkit_4_.28GT4.29
BOINC: http://boinc.berkeley.edu/
Berkeley Open Infrastructure for Network Computing -- Open-source software for volunteer computing and desktop grid computing