OPML is a data structure for all kinds of outlines. It was mainly known for importing/exporting lists of RSS feeds into or from ones RSS news reader.
Hey Homepage plans to incorporate this almost forgotten technique into our Really Social Sites module. So let's say you are following the RSS feed of another website. With the use of OPML we can now also see which feeds this website is following.
Do you see where this is going? It's social media with discoverability on steroids! It's like a blogroll or a webring, but automated.
If you don't have a website from Hey Homepage or Hello Website, you can still take advantage of this resurrected technique. Just offer your blogroll/webring as an OPML file and link to it from within your head tag to make it auto-discoverable.
OPML officially stands for 'outline processor markup language', but we like 'other people's meaningful links' more.