Hobby 3D engines are a dime a dozen these days. I've gone down that road myself, and I can say that it's good programming practice. The R5 Engine is a bit special, though, for several reasons. One, the excellent blog covering its development. Two, that it's made to run on Windows, Mac and (recently) Linux. Three, that it's being used to create a game on top of it. And finally, that all this is the work of one man who I suspect does not sleep at all. :-)
And yes, it's open source. Check the blog for more info.