Only a few scientific instruments have provided more insight that Hubble. Even though it needed corrective optics immediately after deployment to orbit, delaying numerous experiments by more than three years, since 1993 it was nevertheless instrumental in many new discoveries in the astronomy. It has put astronomy to the front pages many times, supplanting celebrities, sports, and even politics. Probably many have their own favourite picture, and even Hubble Heritage Project was started to immortalize this instrument.
Constant upgrades have kept it on the leading edge of astronomical discovery for a full decade. It is probably the single most important instrument created since Galileo's telescope used to observe the Moon and Jupiter. Just like Galileo's telescope is obsolete today, so the Hubble will be obsolete by tomorrow's standards and the science community had been planning for the replacement from the day the Hubble was launched to the orbit. Even in the 1990 the scientist were already thinking about its replacement.
The real question is what will replace the Hubble? When you consider that it took 13 years from the Congress approvals for Hubble in 1977 to 1990 launch, there should already be a project to replace Hubble in the works. Until recently, I had seen nothing about such project, but when I was reading BBC article about a large-format camera that, among other things, mentions the technology used is being developed for the James Webb Space Telescope that is to replace Hubble.
After a quick search, I have found STScI (Space Telescope Science Institute) site that among other things provides basic overview about the James Webb Space Telescope and JWST technical overview of the project. There you can find more information about the scientific goals, current progress reports, targets, simulations and other technical information.
As it turns out, the project conception had already begun in the first years of Hubble and its timing of 13 years 13 years of active development and 20 years expected work coincides nicely with currently planned decommission of Hubble around 2010. Rough estimates leave us with the expectation that JWST will be nearing completion around 2008, since active development started in 1995-1996. Rough estimate for the schedule would then probably be that JWST should be completed somewhere around 2008 (1995 was the year that planning started in earnest). However, the BBC article suggests that launch is planned for 2010.
Since the project is still in the early stages of development, many component designs and ideas are still in a state of flux although main contractors have already been selected and components are being actively developed. Components like the wide-format camera developed for JWST already having an impact on the astronomy now. For those of us used to constant stream of new pictures and discoveries from Hubble this is good news. Science will advance and we can expect more exciting and possibly profound discoveries.
The scientific instruments that will be on JWST will be limited to only three: MIRI, NIRCam, and NIRSpec. The addition of new instruments will be nearly impossible, as the JWST will be orbiting L2 point on a distance of 1,500,000 km from Earth (the Moon is orbiting 385,000 km from Earth). This distance will probably make any service missions impossible, meaning the instruments installed at JWST launch are the only instruments that it will ever have. This is probably the main limitation compared to Hubble that already had four service missions that have upgraded existing, and installed new scientific instruments.