@riking: Usually just the dev console. Type CSS in. In extreme cases, I draw freehand circles using my touchscreen and Paint.
@erlend_sh: Yeh, the dev console and a paint tool should suffice for 95% of all mockups. For most of my mockups I just copy existing UI elements in Discourse and reuse them in new places.
Maybe I’m going too far to look for great tool…For a mockup, there may be no need to take advantage of complicated tools but easily one. So how you make the mockup?