The Nobel Prize committee have made an excellent choice in awarding the 2006 Nobel Prize in Physics to John Mather and George Smoot for their work leading to the NASA COBE (Cosmic Background Explorer) satellite. SciAm has a nice summary of Mather and Smoot's work.
COBE was an Explorer-class NASA mission (*) (i.e. pretty small and low cost compared to HST or Cassini) launched in 1989 that measured the microwave background radiation reaching Earth. This link will take you to a NASA page with more info on COBE.
This radiation arises from warm dust in the Milky Way and also the left over glow of the Big Bang itself, now cooled by the expansion of the Universe to a temperature of about 3 degrees above absolute zero. Detecting this background and showing that it has an almost perfect black body spectrum is pretty much inescapable proof of the Big Bang.
The picture shown here is comes from the COBE data, and shows the separate components to the detected microwave emission:
- The horizontal structure (top two panels) is emission from the disk of our Milky Way galaxy. Filaments and spurs are also Galactic emission.
- The blue-ish curvy structure seen in the top image is microwave emission from inter-planetary dust within our Solar system - the curve actually marks the (physically flat) plane of the ecliptic within which the planets orbit (Pluto's orbit is rather inclined away from the ecliptic, which shows you that its formation or history are significantly different from the other planets).
- The isotropic extra-galactic emission left behind when you remove the previous two components is the T=2.7 Kelvin microwave background left over from the Big Bang, as seen in the bottom panel. The minute variations in the temperature of the background from different parts of the sky also carry cosmologically-important information regarding the shape of space, the mass density of the Universe, and the formation of the first galaxies... stuff WMAP is doing in greater detail now (maybe Chuck Bennett will eventually get a Nobel for his work on WMAP?).
(*) There are several types of Explorer class missions, e.g small (SMEX), medium (MIDEX) and so on. There have been significant changes to the classes of space mission NASA funds since the new Moon/Mars push was imposed, but I'll have to look up the details.