Neat, informative, interesting, or just plain well-written.  Blogs and content on the interwebs that I've loved over the years.

Science: General

Science Base : science writer blog
Atomic Nerds : science writer blog/political commentary blog
RadioLab: an excellent radio show/podcast about wacky science and wacky ideas
Science, Pseudoscience, and Irrationalism: a collection of articles and links
Scientific American Digital:  the on-line version of a wonderful science magazine

Science: Physics and Astronomy
HyperPhysics : basics physics concepts explained and conceptually linked 
SkyMaps: Free on-line monthly star charts
Sky Maps for Your Telrad : special maps useful if you have a telescope with a telrad
The Physics Buzz: official blog of the society of physics students
Astronomy Picture of the Day

Science Culture
XKCD : the best geek culture comic ever 

Environmentalism: General
Environmental News Network

Environmentalism: Climate Change
It's Getting Hot in Here: Not the song, rather a place where young people blog on climate change the largest climate activist organization in the world
Skeptical Science: common climate change arguments listed and discussed

Ethics and Ideas
Your Morals:  this is a series of actually quite involved quizzes linked to a research group that is investigating links between moral perceptions/priorities and political inclinations.  If you participate you add to their dataset, and it's a fun way to nerd out.

Green Building
Building Green Blog : well-researched and in-depth industry blog
Green Building Advisor : another such blog
Energy Vangaurd Blog: another physicist-gone-building-science professional provides a wealth of great commentary on the building performance industry.

Fiction Writing
How to Think Sideways  : a very helpful course and community for aspiring writers.  (Full disclosure here:  this is an affiliate link, meaning I get points if you go there and then sign up for the course.  I only do this because it was an amazing course and I solidly recommend it.)
Forward Motion for Writers : on-line writers community

Fiction Reading
SFReader: speculative fiction book reviews

Gender and Feminism/Masculism
Yes Means Yes Blog:  blog on consent and contemporary dating, feminist style. Note, some content is adult in nature.
No Seriously, What About Teh Menz? Tired of hearing vaguely accusatory "what about men?" arguments thrown into feminist discussion, this group blog was created to generate focused discussion on gender issues from a "masculist", rather than "feminist" point of view, and has matured my own thinking on gender subjects quite a bit.

Linux and Computing
A Beginner's Python Tutorial: at one point I had to learn python, and this was extremely helpful
LinuxChix : community for linux users who happen to be female
VI Editor Cheat Sheet