One of the things I've been looking into recently is how to tweak sites to maximise the conversion rate of visitors. As an example let's look at the birthday brain, this is a site I've developed as a bit of a side project.
The key area of the homepage is the login/register box, the key objective of this page is to get readers to understand the service and register. The secondary objective is to make it clear what the service is about to search engines.
The original design of the login register block looked liked this:

While there is nothing wrong with this design it does hide the register box away under the login box. User's who have already registered will be taken straight to their reminders if they check the remember box and can easily spot the login boxes if not. So making the register button a bit more obvious by swapping the two around seems like an obvious change. This is what we end up with:

A small change but how do we know how big a change it would make if any? I setup the page so a querystring would change which version you were looking at so ?ver=2 would display the new version then setup this in Google's Web Optimizer. This service directs users to one or the other version from google, it then tracks the conversions on the site and shows a graph for each showing the conversion rate. The old version already had a very good conversion rate of 9.28%. The new version however managed to convert 10.7% a very decent 1.42% improvement for an hours work!