<p>I'm trying to get a sampling of what people think are the best sign-up forms.  Good design, usability.  Smart engineering.  Helpful feedback.</p>

One of my all-time fave sign-up forms was the original
Vox
one, which has since been changed; there was
a great break-down
of it published online, and it goes into the things that made it so great to me.  How they implemented the CSS layout of their forms, how they used in-form validation with pop-up tips, etc. -- it was nice.