cocopops

I'm looking for some software I can use to draw reasonable technical diagrams- the sort of diagrams you'd find in online lecture notes, or in maths/engineering textbooks, i.e. simple shapes, arrows, curves etc.

It's hard to describe what I mean, hence why I've came after nothing on Google. For graphs I'll import them from Matlab to the Latex document, so its really just for diagrams which I need to draw myself (hopefully without resorting to pen and paper then scanning...).

Also note, it can be Mac, Windows or Linux software, thanks.