The Phoenix probe to Mars should land today around 6:00EDT. It's a lander designed to dig in the Martian top soil to look for sub surface ice that the Mars Observer's data indicates is just below the surface in the northern polar regions. It will subject this soil and ice mixture to a battery of scientific tests, hopefully which will give us some clue as to the possibility of biological viability in the Martian biosphere.
Update: It's down safe! Congratulations to the NASA team.
I attended the first one, I've pencilled in the next one on my calendar.