I enjoy trying out ideas for web applications and see how it goes.
Bandnews (2004/05), collecting news from the (at the time crazily bad) band websites with a bot.
Blummy (since 2005), in-browser dropdown window for your collection of bookmarklets.
Computerlexikon (since 1999), Fachbegriffe verständlich erklärt. A german computer dictionary.
Colorillo (since 2009), collaborative drawing in the browser.
Conota (since 2014), work in progress. peer review facts.
Factolex (since 2007), create your own encyclopedia.
report a problem (since 2010), a small script that you can drop into your web page and get a in-page feedback form, aimed at getting useful response: what did you want to do? but what happened?
thinkery (since 2011), your brain, organized.
Wizlite (2006), a highlighting marker for the web.