Professional Organizations

• American Physical Society 
• American Institue of Physics
• American Association of Physics Teachers
  

Astronomy

• Picture of the day

Computation /Visualization

• Matlab
  
• Wolfram- Physics
  
  

Resources for graduate school in Physics

• Preparation for Graduate School by the AAPT
• Peterson's Guide to Graduate Schools
  

Physics Miscellaneous

• How stuff works
• Open courses by MIT
• The Nucleus
• Physics Central
• Eric Weisstein's World of Science
• Physics and Astronomy Online