For some reason on all 6 machines where I do development (<GeekMoment>Home machine, laptop and 4 dev machines at work</GeekMoment>) this option is turned off. Not a big deal of course since there are workarounds, but for my side projects (where I actually do UI work) this is actually pretty annoying.

 

This option will make sure all public components and User Controls will magically appear on your toolbox. Very very useful. Wish I knew about it before…

 

It is located in Tools/Options/WinForms designer (bottom option in the list) /AutoToolboxPopulate (bottom option in the grid)