Lutz's CommandBar for .NET
Lutz may be famous for Reflector, but for those of you needing a menu and toolbar library, his CommandBar for .NET is worth a look.
It's not so hot for dynamic menus and toolbars (lots of flickering, which I for one can't seem to fix without breaking it), but if you want a good looking set of menus and toolbars it's certainly easy to use, is reliable and looks good.
One thing - Lutz actually does a rather good job of creating a neat API for his toolbars that follow the standards that Microsoft put out there for .NET developers. Those not looking for menus and toolbars may want to take a peek by way of a best practice example.