I'm authoring two, One that's a keyboard based mnemonic launcher for accessing various websites. It's basically a way to have your bookmarks outside of the browser and quickly accessible. The other is a tabbed markdown document that can render static copies of itself.

The two projects out in the wild that natively work with this approach are TiddlyWiki and FeatherWiki.

I see room for a lightweight version of a calendar, a world clock, and even a lightweight spreadsheet that could be useful. I also have an idea for something I call a link trap where you can rapidly just drop links in and then search and sort over them to quickly retrieve something you saw before that was interesting. Sort of like my bookmarks-outside-the-browser except more of a history-outside-the-browser.