Ice. On Mars.
NASA is reporting that the Phoenix Lander has found ice on Mars.
Actual water ice. When the substance was first uncovered, it was speculated that it might be salt, but it sublimated, proving that it was ice. The Phoenix lander site has an excellent document
explaining why this is such a big deal. (PDF warning.)
Update: Wired has an explanation of how they know it is water ice, and not C02, here