Here's an excellent review from The Telegraph of the events in Ukraine from the February protests to the Russian invasion a few weeks ago.
Ken Halla also posted some great resources on Ukraine on the government blog here, including a great Hip Hughes clip about the crisis.
The New York Times has an excellent overview all in maps and CNN has a good review called "20 Questions: What's Behind Ukraine's Political Crisis.
Finally, Ian Bremmer, an author and president of the Eurasia Group, has an excellent essay in Reuters about who loses in the crisis noting that "if Russia sends its forces into Eastern Ukraine — a distinct possibility — everybody loses. We could see the outbreak of a Ukrainian civil war, crippling market volatility, extreme geopolitical shock, and unforeseeable consequences. Events to date have brought us to a point where this is a frighteningly realistic outcome that cannot be ruled out."