Freakin' cool: the James Webb Space Telescope
Partly as a replacement for the Hubble Space Telescope, NASA is currently designing the James Webb Space Telescope. If all goes well, it will launch in 2013. 
Although I personally think they should keep funding the Hubble as long as it can feasibly continue operating, NASA didn't ask for my opinion, so they plan to let the Hubble burn up in the atmosphere when Webb is launched.
Pity, that. The Hubble is the greatest scientific instrument ever devised, and with a couple of repair missions, it could continue providing data for decades to come.
The Webb is a remarkable device. It operates almost totally in the infrared spectrum and therefore can't see in the optical range like Hubble can; however, its reflecting mirror is six times the size of Hubble's. It is designed to peer back into the heat signatures of the very early universe, and it promises discoveries even more startling than Hubble's. The stuff it will look at is so far away, they've devised a way to let the telescope squint so it can see farther.
The craziest thing about the Webb is its orbital location. This is kinda geeky, in a math/engineering sort of way, but the Webb is going to occupy a very special place, orbitally speaking.
If you consider the Earth and Sun separately from the rest of the solar system, there are five very special orbital locations called Lagrange points. At any one of these five points, an object with relatively small mass (like a satellite or a Greyhound bus) can orbit stably and naturally without any propulsion required. The object is kept in place merely by the interplay of the gravity of the Earth and Sun. I've created the following oh-so-scientific diagram of the Lagrange points to illustrate where they are (I have mad MS Paint skillz):
The Webb Telescope will sit at the point labeled L2. If you were standing floating at that point, you'd always see the Earth located smack dab in the middle of the Sun's disk. With the Webb located at L2, NASA doesn't have to worry about propulsion (other than the propulsion necessary to get it there in the first place). Once it's parked, the Webb will sit at L2 as long as the Earth continues to exist.
The L2 point is about one million miles away from Earth, which is four times the distance from the Earth to the Moon.
If something goes wrong with the Webb once it's in place, NASA has said they'll send astronauts out there to fix it. This would be four times farther from Earth than any human has ever traveled.
NASA unveiled a full scale model of the Webb the other day, which I briefly mentioned here.
The breathtaking technology behind this beast is summarized here.