I have to agree with the praise for TMG. I've been using it for several years now (version 4 then version 6). I have found it to be incredibly flexible in handling conflicting evidence, and it also generates some nice reports. I had moved my data from Family Tree Maker, so it did take me quite a while to "clean up" my database to change from a fact-based system to an evidence-based system, but I think the results have been worth it.
To be clear to the OP, it is definitely not an Open Source program in that it costs money and the file formats are (IIRC) proprietary as well (but there are plenty of export options so you are not locked-in).