The Space Optical Telescope, now known as Hubble Space Telescope (HST) is the optical Great Observatory. It was launched to great acclaim and soon after discovered to be flawed. It's main mirror contained imperfections in it's grinding that resulted from a certain production limitation being accounted for twice. It has now been fitted with the equivalent of spectacles to compensate for this!
The Gamma Ray Observatory, since renamed to The Compton Gamma Ray observatory wasn't so lucky. It's gyroscopes began to fail and when it was down to it's last gyroscope, the choice was to risk losing control or destroying the observatory. NASA ditched the bus sized satellite into the Pacific Ocean in 2000.
X-Rays are also represented in the Great Observatories, with the Chandra X-Ray Telescope. This has been used to great effect to study distant galaxies and is still operational.
The fourth observatory has not yet been named and it's an infra-red facility, due for launch in several years' time.
Other notable space observatories are IRAS, which performed an all-sky survey in infra red, as well as discovering disks of dust and gas around many nearby stars, such as Formahaut, Vega and Beta Pictoris. This ceased functioning in 1982 and has since re-entered.
SOHO is a solar observatory, it is operational and used for the study of the Sun's corona and magnetic environments. SOHO has revolutionised our knowledge of the Sun.
Hipparcos was a satellite for measuring parallax. It revised the Cepheid distance scale to great accuracy and has been invaluable for all branches of observational astronomy by furnishing scientists with extremely accurate "standard candles" for measuring distances.
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