The other option is the perennial favorite, Minix, but I've realized about computing is that there are a lot of thoughts on how to do things correctly and seeing only one way may make you blind.
As an aside, it looks like we've hugged the site to death.
http://pdos.csail.mit.edu/6.828/2014/xv6.html
If you're really looking for a more complete OS for perusing the source, I'd probably say OpenBSD. They emphasize clean implementation and documentation, which makes it a good learning tool as well.
Inferno and the other Plan9 family members maybe aren't the best choices IMO. I haven't done a super deep dive, but my impression is that the communities around them are starved for labor and you might not find things as approachable. There's a lot there to learn of course, but you might have more issues.