I'm working on a few things at the moment which involve drawing circles.
Whenever I have small circular pattern pieces to draw I use this super-useful circle template:
I pinched it from my dad many years ago (what a terrible daughter I am!), it's getting a bit old and worse-for-wear now but my goodness it's a useful bit of kit.
I'm also thinking of buying an old fashioned school compass for drawing larger circles and maybe even a protractor too. Such excitement!