This experiment has been getting quite a lot of airplay in the various science blogs so you may well have heard about it.
This experiment has been running since 1988 by Richard Lenski a professor at Michigan State University and it involved taking a single microbe of ecoli then cloning it into 12 identical lines which were then kept separate. They kept these 12 lines in separate flasks and gave them a small amount of glucose to live off. At the end of each day they took a small sample from the flasks and put it in a new one with more glucose, they did this for the last 20 years.
They postulated that the bacteria exposed to this environment would experience natural selection and evolve, and to verify this he used the clever method of freezing the bacteria every 500 generations so he could always effectively go back in time and follow the course of the bacterias evolution.
So far it doesn't sound very exciting but somewhere around the 30k generation something interesting happened the E Coli evolved a way to eat citrate. Normal Ecoli can't eat citrate it is one of their hallmarks but by a process of evolution the EColi in flasks had found a way to do it. Since they had the fossil record of all the frozen generations before the scientists involved could then go back and track the emergence of this new ability.
What's wonderful about this experiment is that it proves a lot about evolution in a repeatable fashion since we have access to a frozen historical record of the ancestors so can go back to any point and replay things or track the genetic changes that lead to this new ability. The research also shows that it took several changes to gain this ability it wasn't an all or nothing thing like a lot of the anti evolution people suggested it had to be putting yet more nails in the coffin of irreducible complexity.
We have had proof of evolution before in looking at the fossil record and the genetics of related animals but this experiment has basically replicated this in the lab in repeatable fashion with all the history preserved. It basically takes a shot gun to the creationists and ID proponents arguments and shoots it dead. I hope they get the Nobel for this.