I currently use Rackspace Cloud to host a big search engine. (over 40 servers).
What is Akamai, and how does it compare to what I am currently using?
Akamai does content delivery -- their servers are setup to transparently mirror your content, be it HTML/CSS, multimedia, or something else. The benefit is that users get your content from whatever server is closest, so page loads are faster, and bandwidth usage on your server is lower.
It's something of a precursor to the current "cloud" technologies -- does some of the same things, in similar ways. The company's been operating since 1998, well before the "cloud" term was popularized.
I've never used their products, but Joel Spolsky (one of the SOFU founders) seems to be an advocate for them, and that's a pretty big sales pitch that money can't buy. He explains what they did for him here: http://www.joelonsoftware.com/items/2009/02/05.html
Akamai is a Content Delivery Network. If you store your data on Rackspace's Cloud Files system, they have available the Limelight CDN, another CDN like Akamai.