I'll cut right to it: the Advanced Camera for Surveys, a primary instrument on Hubble, is pretty much dead.
It shorted out the other day. Most instruments on Hubble have backup electronics (called "Side B"), but ACS was already using that system because the primary electronics (called "Side A") went on the fritz last year. While the camera isn't totally dead, it is severely crippled. It's main purpose is to take deep, wide-field views of the Universe, and that capability is gone.
Hubble itself is fine, I'll add. This is one camera among many. The Wide Field Planetary Camera 2 is still functioning, as is NICMOS, the infrared camera. STIS, the Space Telescope Imaging Spectrograph, shorted out in 2004.
Right now, there is a servicing mission to Hubble planned for September 2008. It will replace WFPC2 with the Wide Field Camera 3, which will be able to take over a lot of what ACS does (and it can see into the ultraviolet and infrared a little, too). They'll install the Cosmic Origins Spectrograph, an ultraviolet camera, then as well. STIS may get fixed, which would be cool, but it's an intense and intensive repair job. 111 screws have to be taken out, to get to the part to be replaced! Yikes.
Anyway, the servicing mission is booked solid, and there's little chance they can do anything for poor ACS. It has an excellent history -- it took images for the Hubble Ultra Deep Field, the deepest survey ever taken, as well as the COSMOS survey which just released the first ever 3D map of dark matter.
I have many friends who used ACS, and no doubt they are trying to figure out how to work with the data they have, knowing they won't get any more. For some (like Julianne at Cosmic Variance) they can make do, but for others who were waiting to get their observations in, they are out in the cold. This is life in space-based astronomy. You don't get clouded out, but it still has its drawbacks.
The picture above is of two interacting galaxies, dubbed "The Mice", taken by ACS. If it looks familiar, you may have seen it over my shoulder in my video blog entries. I have a framed copy of it given to me after I gave a talk at the Space Telescope Science Institute a few years ago.
This sucks, but honestly, that really is the way things go. ACS was designed for a five year mission, and it shorted out just barely shy of that goal. So we got our money's worth out of it... but that's cold compensation.
To remember happier days, try the ACS website, which has lots of beautiful images from the camera.