Thanks to a blog post by Tim Gowers (https://gowers.wordpress.com/2013/10/24/what-i-did-in-my-summer-holidays/), I discovered TiddlyWiki (http://tiddlywiki.com/).

So far, I've only used it to capture odd jottings, such as notes about references/information sources, ideas to try out, and so forth.  I can see that if I devoted more effort, I could make it much more effective.  In particular, the ability to tag and cross-link items in various ways is very useful.