The Gilbert 21 project provides free biological recording software for people who want to record wildlife and use their records to learn about natural history. You can read more about the background to the project here: https://burkmarr.github.io/Gilbert21/
Gilbert 21 Desktop is a VB.NET application for Windows which started life in 2010 as a personal project by Rich Burkmar. The finished software was made available for anyone to use, but the source code was not, initially available. It became an open-source project in 2017.
Developers should note that this project has not been actively worked on (apart from minor updates) for a few years. It has over a number of years undergone some substantial changes but because the code was not public, it has not been kept very tidy. There is quite a lot of redundant code in amongst the good stuff!