I'm trying to get a sampling of what people think are the best sign-up forms. Good design, usability. Smart engineering. Helpful feedback. Etc.
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.
Is it vain to suggest my own? It's not perfect, but I think it's a good mix of simple, friendly, and optionally thorough:
37signals' Screens Around Town column often has interesting ones. Worth a peek.
I think that Reddit's registration is pretty good. If you attempt to use an action that requires you to be logged in it will pop up in front all Javascripty. It just requires your username and password, and just takes a few second.
There are some nice shots of sign up forms in the flickr set to go along with Luke Wroblewski's "Web Form Design" book
http://www.flickr.com/photos/rosenfeldmedia/sets/72157604272550634/
(which is jolly good - worth picking up if you're interested in this sort of thing).
Surprisingly enough, my all-time favorite, of ones I've encountered in the wild, is Dell's, on their IdeaStorm.
If you click on a control that requires a login (to vote up an idea, for example), it automatically refocuses on the login element. If you don't already have an account you can hit the 'register' tab and no page load is required.
The register form is totally lightweight (four fields I think) and uses AJAX to check if the name is already taken. Once you register you're automatically logged in.
Bottom line, it's visually compact, asks for a minimal amount of information, and lets you login or register without ever leaving the original page.
I like Geni's one (www.geni.com). It's an example of a signup form that doesn't feel like one. You can get started straight away with the site, and are able to add further information as an when you want to.
A couple of examples I find interesting are Tripit, a site for organizing your travel plans. Although there is a link to Sign-up for the service the easiest and quickest way is to forward a confirmation email from a travel service (orbitz, travelocity, united.com, hertz.com etc), doing this will automatically sign you up and get you going (once you log in to the site it will ask for more info).
Another quick and easy registration is Marco Arment's Instapaper. All you need is to fill in your email address or username.