The five second test is a simple usability exercise intended to very quickly assess the clarity of a user interface. It first came to my attention in Robert Hoekman's most excellent book Designing the Obvious (if you're into web app development, get it, seriously get it now).
Hoekman describes it thus:
"To perform a five-second test, write a list of screens on your site or in your application that need to be particularly clear and concise, and either open them in different browser windows (or tabs within a single window) or print them out and bring them to the users. Show each user the screens, one at a time, for five seconds each, and ask the user to write down notes about everything he or she saw."
User Interface Engineering who originally devised the test have a much more detailed explanation in their article which, while lengthy, is actually a pretty good read.
Anyways, this got me thinking that it would be possible to build an online version that retained all of the simplicity of the offline one while avoiding the complexity of gathering people in one location to run the test. With that goal in mind, Jason and I nutted out a version in about 2 days which I think does a fairly good job of replicating the test while making it easier to administer and collate.