Spirit, and her sister robot Opportunity, were only specced to last 90 days on Mars. They have both given seven years of service. Spirit was the one who found evidence that water was at one point abundant on Mars, and gave hope that there might at least at some time have been life there. Spirit eventually developed a bad wheel - she drove backwards for a year or so - and in time dust on her solar panels reduced her ability to respond to NASA. Her ground crew waited for the Martian winter to end, but she did not respond to recent signals, and NASA decided this week to let her go.
XKCD has the definitive requiem, but it is so sad I'll give the second comic, the remix version, first.
Here's a cartoon on how much fun it is to explore the Arean surface. The upbeat one.
And here is the XKCD take on it, bleak but somehow less quotidian.