gcviewer

GCViewer is a free open source tool to visualize data produced by the Java VM options -verbose:gc and -Xloggc:.

It also calculates garbage collection related performance metrics (throughput, accumulated pauses, longest pause, etc.).
This can be very useful when tuning the garbage collection of a particular application by changing generation sizes or setting the initial heap size.

See here for a useful summary of garbage collection related JVM parameters.
For more information on tuning garbage collection on Sun JVMs, take a look at the documentation provided by Sun.