Despite the growing popularity of browser-based office applications, there is one that has always been missing: a simple diagramming tool. Existing Flash solutions have their limitations, so as part of my graduate thesis work I have created an SVG-based HTML5 diagramming widget.
Kanjiriffic is a free and standalone iPhone web-app, designed to help drill kanji using Schultz's KanjiDamage method. It was created when I found the existing free iPhone kanji apps lacking, and wanted a way to quickly drill my kanji when I had some spare time.
At one point in the bad old days of CSS hackery, I created a "mastergrid" showing which browser applied particular CSS hacks (this grid is still available). Collecting this data was tedious enough to get me thinking about automating the process, and later this snowballed into an effort to design an automated browser-testing tool.