Circuit Simulator is built using HTML5 and will run in any modern browser on a reasonable fast machine. You can hook up all the usual components of a circuit including resisters, capacitors, voltage sources, wires etc. and visualize how it works without building anything physical. A great way to experiment!
Google Docs Publisher tries to fix the garbled formatting when trying to publish a Google Docs document to the web. Simply publish the document as usual, then paste the URL into Google Doc Publisher. You will get a new URL pointing to the correctly formatted document.
Gravit is a web-based version of vector design tools such as Adobe Illustrator. It includes the usual pen, line and polygon tools, as well as object alignment/grouping, layers etc. Files can be exported to PNG, JPG or SVG. These file formats can be imported as well.
Resizer lets a designer tests the responsive layout of a website in multiple screen sizes: desktop, mobile, tablet etc. Enter any URL into the address bar and the tool will render that website in a variety of layouts in real-time.
Popcorn Maker lets you easily remix web video, audio and images into cool mashups that you can embed on other websites. You can add media from YouTube, Vimeo, SoundCloud, Audiour etc. The remixed videos will be hosted by Mozilla for free.