I just learned about today.

From their overview page:

CloudFlare protects and accelerates any website online. Once your website is a part of the CloudFlare community, its web traffic is routed through our intelligent global network. We automatically optimize the delivery of your web pages so your visitors get the fastest page load times and best performance. We also block threats and limit abusive bots and crawlers from wasting your bandwidth and server resources. The result: CloudFlare-powered websites see a significant improvement in performance and a decrease in spam and other attacks.

I just set them up for this site under the free plan and plan to set up more.  The set up was fairly easy if you are used to changing DNS settings for your domain.

What are your experiences with Cloudflare?