Ask HN: How does your company A/B test, and roll out new features?

I am approaching a point where I need to test out features safely. I want to know what tools/methodologies exists for rolling out new features and interfaces.

Do you just randomly push 50% of traffic to new features, or is there some sort of routing gradient function that pushes more and more users to a new feature as confidence grows in it?

I was thinking it would be nice to have an experimental web server behind a load balancer, and to slowly push more and more traffic to the server with the new feature before letting all users see it.