From the National Research Council (h/t Insty):
Domestic cats: 100s of millions a year
. Striking high-tension lines: 130 million - 1 billion a year
Striking buildings: 97 million to 976 million a year
Cars: 80 million a year
Toxic chemicals: 72 million
Striking communications towers: 4 to 50 million a year
* Wind turbines: 20,000 to 37,000
If one must have an outdoor cat, the least you can do is to put a bell on the animal.