CloudFlare takes over your domain and reverse proxies your site, to your control. They cache resources for you, selectively, to your complete control. They have some security features, like presenting captchas to dodgy IPs. The base service is completely free, albeit restrictive, but there are no bandwidth caps. They also have "apps" that provide extra features, like asynchronous JS loading, automatically adding Google Analytics to every page, email scrambling, etc. Everything is customisable - if you want, you can completely disable the security features, caching, apps, in fact, you can also disable the reverse proxying for subdomains (which of course removes all the CF benefits).

My web app, http://ponyplace.ajf.me/, has benefited greatly from being on CloudFlare, since it has relieved the burden of serving most static content from my server. It's a really great service, especially for the price. My only complaint is that SSL usage on CloudFlare is pretty pricey.