Updated: 9/25/2018
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]

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]

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]

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]