Back in late September, the Cassini spacecraft which is orbiting Saturn's moon Titan, took an image near the north pole of that moon which is just incredible. It's a 400 km (240 mi) long river. Below is a small part of the image, cropped and rotated:
To get the full hi-resolution image from NASA-JPL, go here. This is a classic dendritic (branching) drainage system as you'd see anywhere on Earth. The river runs into a sea, called Kraken Mare, just off to the left in the image above.
There is a major difference between this river and rivers on Earth, however. The temperature on Titan averages around -180° C (-290° F) and at that temperature, water is solid as rock. Methane (CH4) and ethane (C2H6), however, can exist as liquids, solids, and gases at pressure (1.4 atm) and temperature conditions found on Titan. Below is one model of how this might work:
Strange new worlds. There's some very cool atmospheric chemistry going on here.