The discovery of a new population of star-forming clouds in the halo of the Milky Way galaxy that no one suspected were there, but which now solve part of the mystery of how stars can still form in this rarified region of the galaxy. Also, the recent work with Sasha Kashlinsky and John Mather on setting very tight upperlimits on the infrared component to the cosmic background radiation field in the infrared.
I have also enjoyed the many trips I have taken to Kitt Peak, the VLA and other observatories, and the discoveries that have come from this work out in the field, and just the ambiance of what traveling to these sites and mingling with other astronomers is like.