Unicycle is a free, open source PalmOS app for recording a woman's BBT (basal body temperature) and certain other data that can be used for the "Fertility Awareness Method" (FAM). The current version is alpha: functional, but not perfect.
The instruction manual is available at http://unicycle.sourceforge.net/manual/.
System requirements: You must have PalmOS version 2.0 or later.
Installation: Download the most recent Unicycle zip file at SourceForge. Unzip it. Install "Unicycle.prc".
Source code: Available under the MIT License (that's one of the "yeah, whatever, maintain attributions" licenses). Download the most recent tarball from SourceForge. I use prc-tools (under Linux) to compile it.
Caveats: Unicycle intentionally does not make predictions from your data (that is up to you to do). I have recently noticed another freeware (though probably not open source) application that does offer to make predictions, so if that is what you are looking for, you may want to go investigate "NFP for PalmOS" (with which I have no affiliation).
0.01: First released version.
Unicycle is hosted by: