https://www.3scale.net/ comes with a lot of batteries includes: user login, user dashboard, email handling, if you wish even payment. It's ideal if on the engineering side you just want a simple API call (to 3scale) that returns 'yes/no' for a given API key and everything else can be configured and designed by a non-engineer. We got something running in two days. It's easy to outgrow 3scale though. We're moving away from them because we handle millions of requests/day (saving money).

You might want to check out http://apiaxle.com/. The folks at https://mapzen.com/blog/apiaxle/ seem happy. Near the end of the blog post they point to https://aws.amazon.com/api-gateway/ as well.