Discovery of life on other planets. By life I mean any type of life be it bacterial life on Mars or solid evidence of a thriving ecosystem on a distant planet.
Bacterial life on Mars is still definitely not out of the question, especially with the relatively recent discovery of methane in the atmosphere. Discovering life on another earth life planet orbiting another star is I believe only a matter of time. Recently they made the first spectroscopic analysis of the atmosphere of an extrasolar planet. Combine that with probes to be launched soon that should be able to find earth sized planets around nearby stars. It will take some time but the technology to find evidence of life on these planets is already here, it's just a mater of time and money. Whether it will happen in my lifetime or not is another mater, I hope so!