Russia caused headlines across the world when it demanded that ukraine pay market prices for the natural gas exported by Russia, increasing the cost four-fold. Ukraine refused, and on New Year's Day, 2006, Russia shut off the gas.
Most media sources reported that Russian President Vladimir Putin was using gas to leverage influence in the former Soviet Socialist Republic, thus turning off the gas was a way to punish Ukraine for the humiliation Moscow recieved during the Orange Revolution.
Some, however, think that there's more going on than that...
Related Link:
“Russian-Ukrainian Gas Deal—What’s Behind It?“ article from the European Tribune