The Horizontal Form
Here I've made each fieldset extend the full width of the page. The main styling that's going on is the positioning of the form elements within the fieldsets. Because each element is contained inside a label tag, you can just float labels left, give them a bit of margin, and they line up horizontally (moving to the next line if there's not enough width).
Because I'm using floats, this form will look funky in some browsers unless you hack around with the code. Here's a page that uses clearing divs to make the fieldsets expand to fit the elements within.