June 2011 - Chris O'Hara - (archived original post)
Rate limiting can be an effective way of conserving resources and preventing automated or nefarious activities on your site.
A common use case is to limit the amount of requests an IP can make over a certain time frame. For example, you might want to restrict users from using an expensive search utility on your site. If the user attempts to search more than 5 times a minute, you can redirect them to another page informing them that they need to wait.
IP based rate limiting is already in use on larger sites. Google and Yahoo both employ the technique to prevent (or at least complicate) automated requests to their services.