According to numerous news articles, a Malaysia Boeing 777 has been shot down over Ukraine. The flight reportedly had 295 on board. Ukrainian President Petro Poroshenko said that his country’s armed forces didn’t shoot at any airborne targets. “We do not exclude that this plane was shot down, and we stress that the Armed Forces of Ukraine did not take action against any airborne targets,” he said. “We are sure that those who are guilty in this tragedy will be held responsible.”
A quick analysis of data viewable at FlightAware.com shows the flight departed Amsterdam a half hour late, at 12:30PM local time (Central European Summer Time, CEST). The flight was planned for 11 hours and 28 minutes flying time, to arrive at Kuala Lumpur at 5:59AM local time. No route plan is available at FlightAware, but the total direct distance is noted at 6,368 statute miles.
The flight on the day before (7/16/2014) appears to have flown through the same location, slightly south of the incident route, crossing the length of Ukraine and over the Sea of Azov.
The flight profile shows a normal climb, a long cruise at FL310, then a climb to FL330 and a level off for less than ten minutes. The FlightAware data viewed with the earliest news reports showed a long series of positions, without altitude encoding. Later data presentations showed a 68-minute gap, from the position in northwest Ukraine where tracking first appeared to end, to a single lat/long position near Donetsk, at what appears to be the vicinity of the debris.
Here are a few maps and satellite views, from FlightAware: 
This picture of a portion of the fuselage is said to have landed near Donetsk which is in eastern Ukraine, near the Russian border, and hundreds of miles east of where the flight data ended in northwest Ukraine. An online video shows streaming and fluttering debris in front of a background of black smoke.
NOTE: in the first hours while this news story was breaking, the flight data depicted on FlightAware ended in western Ukraine, at approximately latitude 51.20N and longitude 25E. The flight data was eventually updated, to show one more position more than an hour later, in the vicinity of the debris field. Here is a portion of the World VFR chart, with an orange circle at the Lat/Long.