Hacker News: Frequented by all manner of programmers, developers, sysadmins, designers, engineers, whatever and probably all being about as "social" as they will ever get. Much of the content is way over my head but there are always items of interest.
Hardfought: This is my gaming home on the internet. They have servers on three continents and currently offer 18 different flavors and/or variants of the venerable roguelike, NetHack. You can even play the current developer's version before it's ready for release (or deemed stable).
Internet Archive: An exceedingly deep, vast and varied resource from which you may never return. Which is not necessarily a bad thing.
KMFA: This is a non-profit, listener-supported classical music station that has been broadcasting for over 50 years. They also stream their content for the web. I listen to it incessantly.
Markdown: Very much in keeping with the spirit of my site. Markdown is a simple text format used to create web pages by applying the markdown perl script. Delightfully light-weight and easy enough to be useful almost immediately. And it reads like a book. This page was created by markdown.
Metafilter: A moderated social site that's been around for over 20 years. Known for the quality and diversity of its content. And unlike elsewhere, be sure to read the comments. Much more info available from Wikipedia.
NPR Text: National Public Radio's text-only news site has the same content as their main site but consumes far less bandwidth. Pops up in a second. I believe it originated post-9/11 as a response to the heavy network traffic that created and has remained valuable and popular ever since.
SDF: A nod to my gracious host. I've been messing around here on the Super Dimensional Fortress for several decades now, under one guise or another. Wikipedia provides some background for the uninitiated.
TRIMREAD: A handy little site for circumventing various things bothersome. Works on just about all. It even provides a permanent URL for the converted item. Paywalls be gone!