BoliMonitor is a very simple MacOS X program I wrote when the due date for our daughter was approaching, for my wife to keep track of contraction durations and intervals. In the end, the program went unused because our daughter was born through a medically-indicated C-section, but it’s functional, so here it is.
Just press the button whenever a contraction starts or ends. The running timer on the right shows the length of the current contraction/interval. At the bottom you can see the lengths of the last contraction/interval, the average of the last three, and the overall average. The “Stop” button stops the running timer and discards the current contraction/interval.