The best OS's I've seen for beginners are SteveOS and JeffOS...both are really easy c code on setting up the IDT, keyboard handlers, basic memory management/paging.

For something more serious, I really love ReactOS (I'm a developper in the team). It's an open-source clone of Windows NT and I'm sure in a couple of years will run more then 80% of Windows apps. The whole kernel is really enlightening to look at, and it works basically just like NT's.

Best regards,
Alex Ionescu
http://www.relsoft.net