As a small follow-on, there is also Barrage [1], which is more of a framework than anything else. kellabyte wrote it to benchmark changes to Haywire [2], so it's geared towards configuring your client and server, and then collecting all of the metrics from both the client side, and the server side, and displaying them in an easy-to-digest format.
This obviously isn't terribly useful if you're just trying to gauge request rates, or apply load, but it's very useful if you need a repeatable way to not only measure request rates but see how your server is doing under load.
However, I'm easily tempted by the new and shiny which led me to find another great HTTP benchmarking tool: https://github.com/tsenart/vegeta