Would like to highlight that this webpage, interactive 'screenshots' and all, is 31kb
Was suspicious of how high quality the screenshots were in mobile, but didn't bother to click on them. Neat!
I also use my browsers' DevTools for faster CSS iteration, and Paint.NET for concept art and comparing/measuring screenshots. Sometimes I even use Illustrator and/or Inkscape for some SVG stuff (which I manually edit in code afterwards) - in this blogpost I used Inkscape to recreate the cat emoji and the Chrome iframe error pixel art icon.