CloudFlare wants to provide a WAF to a very large number of customers.
I recently had a problem where a Magento store would infinitely redirect, from TLD to sub-domain (i.e. non-www to www). The server was configured behind a reverse proxy, which was handled by Pound (acting as a load balancer and SSL wrapper). Pound is great for handling hand-overs between a caching proxy and application servers, in addition to load balancing multiple servers and wrapping SSL connections to the client.
I was working on a project which was randomly failing to load certain views outside the development environment. It turns out that the system was running out of memory. After searching some of the error messages output by the script, I stumbled upon the xhprof PHP module. It was originally created by Facebook, and released under an open source license.