Astronomy & Space:

• http://antwrp.gsfc.nasa.gov/apod/astropix.html
• http://blogs.discovermagazine.com/badastronomy/
• http://www.universetoday.com/
• http://www.bautforum.com/
• http://www.spaceflightnow.com/
• http://www.thespacereview.com/

Electronics and hacking/DIY:

• http://hackaday.com/
• http://www.evilmadscientist.com/
• http://www.atmel.com/ - makers of the AVR 8-bit microcontroller, as well as a variety of other 8 and 32-bit microcontrollers, radio transceivers, etc.
• http://www.avrfreaks.net/ - forums and online community specializing in the AVR microcontroller
• http://www.sparkbangbuzz.com/ - Homebrew cathode ray tubes and other home-made electronic components
• http://www.teralab.co.uk/ - physics, electronics, glass-blowing, other neat stuff

Electronic part sources:

• http://www.digikey.com/ - Huge selection, typically available in small quantities at decent prices. Decent parametric search to help you find what you're looking for. Slightly prone to giving you a properly labeled bag containing the wrong part.
• http://www.mouser.com/ - Similar to Digi-Key, wide selection, good search facilities, sometimes better prices.
• http://www.radioshack.com/ - A pale shadow of what it used to be, but they've added some new stuff recently. Overpriced, but they have stores spread across the US. Good if you need a cheap part or two and don't have time to stick it in an order from someplace else.
• http://www.mpja.com/ - Lots of power supplies, not so good for ICs. Other stuff too.
• http://www.jameco.com/ - Somewhat more of a selection of ICs. Slow and annoying website.
• http://www.sparkfun.com/ - Lots of DIY/hobbyist electronics stuff. Kits, breakout boards for otherwise difficult-to-use surface mount parts. Tutorials and active forum community as well. Can be expensive, but good for prototyping or stuff you wouldn't otherwise be able to use.
• http://www.goldmine-elec.com/ - Some cheap parts, lots of oddball stuff. A rather chaotic selection.
• http://www.sciplus.com/ - ...and I said Electronic Goldmine had odd stuff. They don't have much in the way of electronics parts, but do have motors, fans, optical components, cases and containers, lab supplies, and...stuff.

App notes/white papers/other information:

• http://www.atmel.com/dyn/products/app_notes.asp?family_id=607 - mostly AVR-specific, but some more general stuff
• http://www.analog.com/en/content/RAQ_index/fca.html?ref=ASC267
• http://www.analog.com/en/technical-documentation/resources/index.html
• http://focus.ti.com/general/docs/techdocs.tsp?siloId=1
• http://www.fairchildsemi.com/apnotes/

Graphics:

• http://www.povray.org/ - free raytracer
• http://radsite.lbl.gov/radiance/ - another free raytracer, oriented toward lighting simulation and HDR applications
• http://jedi.ks.uiuc.edu/~johns/raytracer/ - a fast and simple raytracer, highly cross-platform and with support for parallel/multiprocessor rendering
• http://www.blender.org/ - 3D modeling program
• http://www.clockworkcoders.com/oglsl/ - OpenGL GLSL shader tutorials and libglsl library simplifying the use of shaders
• http://local.wasp.uwa.edu.au/~pbourke/ - Paul Bourke's website...some POV-Ray, some 3D, some general math, lots of very interesting stuff