Simple but very good generic GIS software, very useful to biodiversity scientists in general to get a good view on the spatial aspects of their data. Also, it is really oriented towards environmental issues and modelling, so it can at the same time stay relatively easy to use and provide advanced features in these topics. It's also a freeware. Windows only, but seems possible to use it on a mac using wine/winebottler. Free spatial data and help forum are also provided on their website.