The planet Mercury is the least-well known body in the inner solar system. The only fly-by mission until now was Mariner 10, which flew by Mercury three times in 1974-1975 (and each time it passed it took pictures of the same part of the planet).
We will be learning much more about Mercury in the upcoming years, as the NASA probe MESSENGER will be flying by the planet three times; twice in 2008 and once in 2009. Each time it passes Mercury, it will slow down, which will enable it to enter an orbit around the planet in 2011, with at least one year of orbital observations after that.
The first fly-by is on January 14, 2008, so we should be seeing the best images we have ever seen of the first planet this week.
Mariner 10 image, NASA
Grace and Peace