Question: I notice a lot of sites host their resources on a separate domain from the main site, e.g. StackExchange using sstatic.net, Barnes & Noble using imagesbn.com, etc.

I understand that there are benefits to putting your static resources on a separate host, possibly with an efficient static-file web server like nginx, freeing up the main server to focus on serving dynamic content. Similarly, outsourcing to a shared CDN like cloudfront Akamai is logical.

What is the benefit to using a separate domain otherwise, though? Why sstatic.net instead of static.stackexchange.com?

**Update**: Several answers miss the core question. I understand that there is benefit to splitting between multiple hosts — parallel downloads, slimmer web server, etc. But what is more elusive is why multiple *domains*. Why sstatic.net rather than static.stackexchange.com as the host for shared resources? So far, only one answer has addressed that.

Answer: 
Many sites have quite a lot of cookies set, these cookies have the purpose of supporting some kind of state.

By putting the static (stateless) resources on a completely different domain you can reduce the size of the http requests. In some cases there are so many cookie that a single http request takes two TCP packets to be transmitted. So having a separate domain is one of the ways to reduce the number of packets to request the various parts of a page. 

Other methods with the same goal are merging a lot of images into a single sprite and merging all Javascript into a single file.
Lèse majesté covered the main points, but to expand further I'd add that having a single domain for all the various Stack Exchange sites means that someone browsing them will only download static content such as JavaScripts once. Going to Superuser, for example, a user will use the cached content since it is from the same place.

There's some more useful info on [Yahoo](http://developer.yahoo.com/performance/rules.html#cdn) and [Google](http://code.google.com/speed/page-speed/docs/request.html#ServeFromCookielessDomain) about this.