Browsing through some of the SPC2011 conference materials, I came upon a useful website that provides usability testing from the perspective of how a user would interact with your web design. You simply provide a web design image or HTML I believe and a series of tests are run to assess how your design is used by real-world users.
The click responses to questions about the design are grouped and displayed
so you can see whether what you thought was a prominent design for something
important actually does the job of attracting user interest.
It's not a free site, but a test is only $9 (about £5.70 at today's price) which is possibly well worth the
pay-out to get some useful design feedback upfront of any development work.
Check out http://www.intuitionhq.com.