[UPDATE: The launch of GRAIL has been postponed once again to Saturday, tomorrow. There are two launch windows; one at 12:29:45 UT and the other at 13:08:52 (08:29 and 09:08 Eastern US time). The weather forecast is iffy, so there may be another postponement. Stay tuned.]
NASA's GRAIL mission -- Gravity Recovery and Interior Laboratory, that is -- was supposed to launch on September 8, but winds prevented takeoff. They will try again Friday (tomorrow) at 12:33:25 UTC (08:33:25 Eastern). Failing that, another launch window will be at 13:12:31 UTC (and lasts for only one second!). There are many more opportunities to launch until October 19, so I suspect NASA will play it safe. Emily Lakdawalla, as usual, has details.
GRAIL will head to the Moon, and is actually two separate spacecraft, each about a meter on a side. They will fly in formation, and use a suite of very sensitive detectors to essentially determine their distance from each other. The Moon is lumpy in its interior; in other words its density varies on the inside. This means an orbiting object will feel a slightly different pull of gravity with time as it circles the Moon. That change in gravity will change the orbital speed and therefore the distance between the two probes. They will be able to measure their separation to an accuracy of just a few microns... for comparison, a human hair is roughly 50 microns thick!
This will allow scientists to measure the interior distribution of mass inside the Moon, essentially probing its interior. The spacecraft also have cameras to take pictures of the Moon's surface that will be available for schoolchildren, a project I think is very cool. But it'll be a while: it will take a few months for the spacecraft to make their way into lunar orbit. They're taking the scenic route, which saves fuel and allows a thorough checkout of the spacecraft.
You can watch the launch live on NASA TV.