And for your continued reading, searching for "DMX Software" in the search bar above produces countless threads where we've already covered this extensively.
ChamSysMagicQ isn't actually "open source" but it is free and very powerful. You'll find it has a fairly steep learning curve, but you can do a lot with it. You'll also find fairly quickly that you want to spend the $1000 or so to purchase one of their add on wing panels. But that's how they make their money.
What are your plans for it? If you are going to be running lights at a party from your laptop the MagicQ is overkill. However if you really want to learn about lighting control and have a brilliant bit of free software then MagicQ is yer man for sure. It is proper grown-up software, the same as is used in their consoles.
I've really enjoyed using freestyler, it's fairly easy to use once you read the manual and powerful enough to do some cool stuff with it. Completely free, I suggest a enttec prodongle for it.
I'll second D::Light. It's surprisingly powerful, free, intuitive (though the manual is not that great), and has fantastic Midi functionality so you can run everything off of Qlab, for example, on one computer. I've been using it for a bunch of stuff recently. It does require the EnttecDMX USB Pro, though.
It doesn't really deal with devices, though. (I think that's coming in a later release)