The National Space Society Space activist group focusing on legislation and commercialization. Headed by Buzz Aldrin.
The Planetary Society Another group I'm a member of which leans more towards public education and non-government/commercial projects (such as SETI). Co-founded by the late Carl Sagan.
SETI@Home Search for Extra-Terrestrial life from home. A project run by SETI and The Planetary Society using screensavers on PC's throughout the world to search for alien radio signals.
Countdown Creations Your ultimate store for astronaut autographs, spacecraft replicas, astronaut costumes, NASA logo clothes and gifts, and items flown in space.
Trans-Lunar Research A non-profit company working towards a privately funded lunar base. Founded by members of the
Pacific Rocket Society.
The First Millennial Foundation A group founded to help save Earths environment and to colonize space. Founded by Marshall T. Savage, author of the book "The Millennial Project: Colonizing the galaxy in 8 easy steps.
ASTRONOMY Magazine
Lunar Institute of Technology Discussions and ideas on interstellar travel from the viewpoint of a futuristic college on the Moon.
The Case for Mars Info on a conference on the exploration and colonization of Mars. Has lots of links to images of the Red Planet.
Extrasolar Planet Search Info on extra-solar planets from the discoverers of 3 of the recent planets found.
The Harvard - Smithsonian Center For Astrophysics Covers all the research projects being run browsable by topic.
The Near Earth Object (NEO) Program NASA's program for finding near Earth asteroids and comets.
LunaCity Lots and Lots of Space, Science and Celtic links.
The Nine Eight Planets Detailed descriptions and photos of the eight planets and many of their moons. Also includes many of the major asteroids and comets. Has the mass and size of all the bodies as well as interesting information such as where their name comes from.
TerraServer Microsoft's test database of satellite imagery. Has a decent portion of the US and bits and pieces of other countries. You can spend hours scrolling around thru the images.