AIP, the American Institute of Physics headquarters are located at the American Center for Physics, College Park, Maryland. They and their member organizations offer programs, publications and services to its members.
You can find the AIP web site here.
APS, the American Physical Society publishes journals such as The Physical Review, Reviews of Modern Physics, Physical Review Letters and a host of other research publications.
You can find the APS web site here.
AAPT, the American Association of Physics Teachers (as the name implies) serves the physics teaching community at the school and college and levels.
You can find the AAPT web site here.
SPS, the Society of Physics Students brings physics resources to college physics programs in the form of visits by leading scientists, scholarships and awards. SPS also sponsors Sigma Pi Sigma, the National Physics Honor Society.
You can find the SPS web site here.
More Cool Links
Physics Simulations - Try these simulations from the Physics Education Technology Project of the University of Colorado.
You can find the PhET web site here.
NASA - There's always something going on at NASA.
You can find the NASA web site here.
The Hubble Telescope - Check on the space telescope and Hubble's future.
Hubble's web site is here, and more - including fun stuff) here.
Physics Jokes - (Most of them, well, not great...)
Find the jokes here.
And, finally, more physics fun stuff - With more links
Find them here.