A popular one that works well is Cloudflare:
Another option is MaxCDN:
There are a number of articles you can read on MaxCDN vs. Cloudflare
Some top CDNs that are expensive are EdgeCast and Akamai.
Another option is Amazon's CDN:
Many of our clients use either MaxCDN or Edgecast.
I agree with all the providers listed by @robfico except CloudFlare.
We have seen many users facing slowdown after using CloudFlare. This may be due to CF attracting users who are prone to DDoS attacks and as such when their network is being DDoSed, everything slows down including their CDN.
Cloudflare for free choice
MaxCDN or Amazon Cloudfront for paid choice. But you may use free choice of CloudFlare what 's really good too.
To me, the easiest one is CloudFlare.
the question is - why you need it? CDN is good when your shopers are coming from around the world.
but if your visitors from UK, then your media will make an evil round trip...
Amazon Cloudfront for us, it gives us much more control over what we deliver via the CDN, and how
If you do not need all of the features of a full fledged CDN and just want to move your */media* to a CDN then check out ImageCdn from OnePica.
It is a free opensource module that will allow you to move your media to Amazon S3, Coral CDN Rackspace Cloudfiles or others.
Using Amazon S3 Reduced redundancy it is as little as $0.0312 per GB and $0.0004 per 10,000 GET request. this is by far the cheapest option I have found for lower traffic sites seeking a speed boost.
My vote would be for Amazon CloudFront. I've used it for years, it's pretty easy to setup, good value, and works really well.