[NicktheGeek]

This looks liek a custom post type with a custom archive template and some basic :hover CSS to alternate between one div showing and another div showing. I'm not aware of a plugin that does this specifically, but the CSS isn't really difficult. Basically you have

<div class="cool-hover-effect">
    <div class="normally-shows">
        Add whatever you want to normally show, be sure to include classes to make it look pretty
    </div>
    <div class="shows-on-hover">
        Add whatever you want to show on hover, be sure to include classes to make it look pretty
    </div>
</div>

Then your CSS looks like

.cool-hover-effect .normally-shows,
.cool-hover-effect:hover .shows-on-hover {
    display: block;
}

.cool-hover-effect:hover .normally-shows,
.cool-hover-effect .shows-on-hover {
    display: none;
}

That site isn't doing it but this is where I'd also make sue of a CSS transition so it does a nice slide or fade transition on newer browsers.