Zurb’s Clue is another great 5 second test service. They describe it as a fun and easy way to test what people remember on your website.
To use, enter a URL for a page you'd like to test, they generate a screenshot, and provide you with URLs for the test and the test report. Here's an example using this site: test and report. Feel free to mock my home page at will. ;)
Users will see a screen with these brief instructions, "Look at this screenshot and remember what you see," and upon clicking the Take the Test button, they are flashed the screenshot for 5.5 seconds. Afterwards they're given 5 inputs to describe what they remember seeing. Go to Clue App to create your own.
If you're interested in learning more about this type of first impression task, read about it UIE's article about the technique.