Joel Ward is a light-scoring winger for the Capitals, who was in the right place and the right time to put home the overtime series-winner to send Washington past Boston in game 7. Completely unrelated, for most of us, is that Ward's parents were Barbadian immigrants to Toronto. So there were a good number of viewers who didn't see a Capital, or an unlucky rebound, or the heartbreaking flukiness of the playoffs—they saw a black guy.
There was a ton of this last night. (Someone compiled a more comprehensive list at Chirpstory, including many people who later deleted their Tweets or their accounts altogether.) And it's not some bullshit about how the anonymity of Twitter encourages people to say things they'd never say in real life—someone who's go-to insult is "nigger" is the type of person who does say this stuff in real life. Now, because of Twitter, we all get to hear it.
There are lots of backstories here, if you want them. Boston's troubled history with racism. The fact that it was the Bruins who employed Willie O'Ree, the NHL's first black player. The fact that Ward scored on Tim Thomas, self-described "free citizen" and Ron Paul fan. But we don't really need the sociologists. This is just a bunch of people whose first reactions to Joel Ward scoring a goal were to call him a nigger.