Updated: 9/25/2018
Graphics-online

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Draw.io (Upd: 7/7/2016)

Draw flowcharts online. And print them. Export them. Has mockup buttons and objects. This now redirects to Google Drive. [Report a dead link]
https://www.draw.io/

Flock Draw (Upd: 4/25/2016)

Kept crashing on Firefox 43.0, 46.0. Ok on Chrome. Very basic drawing tools. [Report a dead link]
http://flockdraw.com/

Google Drawing (Upd: 4/25/2016)

It's part of Google Drive. LOTS of shapes here. Many shapes for software developers, flowcharts, electronics, mfg process, even has lines with arrows. [Report a dead link]
http://drive.google.com

Method Draw (Upd: 4/25/2016)

Draw SVG graphics online and download to desktop. It also shows SVG source for other uses. Save as PNG. [Report a dead link]
http://editor.method.ac/