My favorite was Cloudflare because I had a similar idea years ago that I was going to do as a startup, but chose another idea instead.
The tl;dr version of it is a forwarding proxy for your website that speeds up page delivery (gzip, etag etc), filters out vulnerability and exploit attempts, filters out spammers, filters out mis-behaving bots etc.
I am not sure if Cloudflare is application aware. The way that I was going to do it was to build modules for popular web applications and optimize based on that (eg. it would recognize Wordpress and compress the JS/CSS, sideload images, etc.)
Clickable link: https://www.cloudflare.com/
That is an impressive website (even with flash disabled) and a welcome change to the standard screenshot/header/sub-menus/links in footer boilerplate that we see too often.
(As an aside, the winner, Qwiki, is co-founded by Louis Monier - link for those who don't know him or don't recognize the name: http://en.wikipedia.org/wiki/Louis_Monier)