SPACE
Space Telescope Science Institute
Spitzer Orbiting Telescope
Marshall Space Flight Center's
 
J-Track 3-D
(find out where over 500 satellites are right now)
Fourmilab's
 
Solar System Live
(find out where the planets are right now)
JPL's
 
solar system simulator
SpaceWeather.com
NOAA's Space Environment Center
has a service called
Space Weather Now
NASA
Today @ nasa.gov
Great Images in NASA
Earth Observatory's Image of the Day
Science@NASA
Visible Earth (a NASA directory of images)
Space Shuttle Earth Observations Photography database
Latest images from the Multi-angle Imaging SpectroRadiometer
KSC Photo Corner
Tropical Rainfall Measuring Mission (TRMM)
  has a map of the  
global precipitation from Jan 98 to the present
Ulysses: space at high solar latitudes
STEREO's movies of the sun
Other Space Agencies:
ESA
Solar System Division of ESA's Space Science Department
Canadian Space Agency
Brazilian Space Agency
National Space Development Agency of Japan
German Space Operations Center
Italian Space Agency
United States Space Command
Space pictures from Australia
The Solar Fleet:  
SOHO
 
TRACE
 
POLAR
 
ACE
 
IMAGE
 
CLUSTER
 
WIND
 
GEOTAIL
Space Reference.com