We don't know how many people are killed by police. That's an outrage. But seriously, think about it: police kill more people than America executes. We spend a lot of time and money when it comes to executions. And we don't even count those killed by police. And this isn't even a pro- or anti-police issue. Either way we just need to know.
Here's somebody who is taking it on himself to keep track. It might even work. Similar methods have been used to give us idea of how many people have been killed as a result of fighting the drug war in Mexico. Another website keeps very good track of Taser-related deaths since nobody else does (about one per week, in case you were wondering. And here's a parallel site).