Planetary scientists have suspected for quite a while that Titan, one of Saturn's moons, has a 'methane cycle' that's much like the water cycle we have here on Earth. New pictures from Cassini suggests that the similarity between Titan and Earth may be even more pronounced, as astronomers have identified a huge, Earth-like river system full of liquid methane.
This river on Titan's surface is about 200 miles long, and it's definitely got liquid in it, showing up black (which means smooth) in the radar image. Water would freeze on Titan's surface, but methane and ethane are stable liquids, and thanks to Cassini, we know that there are huge lakes of the stuff down there. What was less certain was how the lakes filled themselves, and it seems to be a combination of much of what goes on here on Earth: rain, underground aquifers, and river systems. Here's a gigantic pic of the as-yet unnamed river flowing into a lake (at the top of the image):
For the record, this is the largest and most complex active river system ever seen anywhere besides Earth, and as far as we know, Titan is the only other planetary body with stable liquid on its surface.