The Jargon File
The Jargon File's old homepage seems to be unceremoniously redirecting visitors to random Linux-oriented sites, so the link goes to a mirror. If you've not heard of it before, it's "a comprehensive compendium of hacker slang illuminating many aspects of hackish tradition, folklore, and humor." Taken with a grain of salt, it's fairly interesting, and you can easily get lost in it for hours. I particularly like the AI Koans and A Story About Magic.