Check DNS resolution speeds from various places around the world, very useful.
In a similar vein to cloudflare / yottaa, strangeloop offer a performance enhancement service through a physical / cloud based appliance. It uses the standard techniques, js/css combining, sprites, cdn, tailoring to the browser, and so on. No pricing. :-(
Similar to CloudFlare but more expensive ($30/mth no ssl, $130/mth with ssl) it accelerates web sites through the use of CDN, js/css combining, etc.
loudFlare is an automated CDN / content acceleration type service. It acts as a reverse proxy providing some sort of heuristic security scanning, logging of traffic, and an automatically configured CDN. $15 + $5/site/month.
CDN plugin for Magento that supports Rackspace and Amazon AWS CloudFront. Pushes images on demand, claims to be simple to integrate, and is free.
1TB of data for $39 (limited offer at 18 Mar 2011). Hat tip Anthony Feint. Plugins for drupal, wordpress, etc. Data is around 10c/gb up to 50TBs, comparable to EC2.
Limelight networks offer CDN (Content Delivery Network) services and other streaming and static media hosting.