Even though I know there not compareable, opml and topic maps. I will use them in simular situations. Or at least write down notes using a outline tool which supports opml then convert them into topic maps.
I'm finding more than ever the need to keep a track of the actual relations between different things. Not just the things themselves.
Good starting points
Getting started with Topic maps
Xml.com's Introduction to topic maps
Company selling topic map software and a free topic map viewer, they also have some ok papers on topic maps. Including topic maps, rdf and other semantic languages