Does anybody know a good free software for whiteboarding and quick sketching?

I tried Leonardo and it's actually pretty damn good but is not free at all, tried Mischief and has some really bad performance issues (plus is not maintained anymore), tried Milton but is still immature, anything else (OneNote, Gimp, Krita) is just not what I need.
I want it quick, lightweight and easy to use just like Leonardo, but free.

So much was my disappointment that I decided to create my own sketching software from scratch while studying...