With the launch of their Android version, I became aware of CyberTracker, a free "GPS Field Data Collection System that turns your smartphone into a sophisticated tracking and monitoring device". It's a non-profit organization and they mention free software several times, but not certain that is open source software, anyone knows?
From the website: "You can use CyberTracker on a Smartphone or handheld computer to record any type of observation. CyberTracker, which requires no programming skills, allows you to customize an Application for your own data collection needs. […] CyberTracker is the most efficient way to gather large quantities of geo-referenced data for field observations at a speed and level of detail not possible before. Observations can be entered with a simple Radio List or a Check List. Number and text fields can also be entered by means of conventional key pads or keyboards."
Plenty of screenshots here and the list of features.