^I second the Sigma 120-300 f/2.8. I have one and I've used it at my job (professional indoor football) and also at some concerts, MLB, and NFL games.
I think it's very, very underrated. The AF isn't quite as quick as a Canon L lens, but it works very well for me at indoor football, which is very fast paced. It's sharp and very versatile. You can buy a USB dock for it that allows you to customize your AF settings on your computer.
It gives you a similar focal length on a full frame body that a 70-200 would give you on a 1.6 crop body. Not to mention, at $3600, it's about $7k less than you'd expect from a Canon or Nikon lens. If you're using a 1.6 crop body, the Canon 135mm f/2L (around $1k) might be a decent choice. It'll give you solid reach on a crop body and the f/2 aperture is awesome to have in low light.