I'd really recommend setting up a virtual machine and looking at code and binaries from the links on the
project list. 90% of projects are really not far along enough to support much concurrent development without very close contact. Just try and find something you like and contact the head developer of that project.
This is a surprisingly uncommon question - most people who start out make one fifth of a simple OS and try to advertise it to other members (with little success). Free developers are in extremely short supply, so expect a lot of people advertising their OSes on this thread.
(Obviously, due to ego reasons,) I think you should check out my project as well. I'm not quite ready for other developers, but within the next few weeks I expect the kernel to be stable (it works now, but there are a couple of bugs related to the driver API), at which point drivers can be written. It's a microkernel, so driver development is very abstracted from the rest of the system. What I really need help with is graphics, specifically creation of a good graphics API. If you're interested, my git repo is in my sig and my email is in my profile.