Bart Smaalders has written some great stuff about event ports including an extensive coding example. Event ports provide a single API for tying together disparate souces of events. We had baby steps in the past with poll(2) and select(3c), but event ports let you have the file descriptor and timer monitoring as well as dealing with asynchronous I/O and your own custom events.

Corporate shill that I am, there's now a little article I wrote on the meet the architects page. The DTrace team already has a column as a group, but in this one I focus on application tracing which was my primary contribution to DTrace.