Ask HN: Your "Keeper" articles.

This being Hacker News there are many articles here that are "of the moment," items absolutely worth reading, and interesting to see, but not ones that you would refer back to.

There are articles, however, that are worth filing away and coming back to read later, and I'd be interested in seeing the ones that others have kept hold of in this manner.

Recently we've had "Who Can Name the Bigger Number?" ( http://news.ycombinator.com/item?id=1539538 ) as an example, an article that repays rereading, and I personally enjoyed Zed Shaw's "Ragel state charts" and PG's "The Top Idea in Your Mind" ( http://news.ycombinator.com/item?id=1536356 )

I've filed those away with some tags to help me find them again. What articles are your keepers*?