A user will fill up the registration page of a site when he/she wants to access parts of a site or application. This is a long time challenge to design a form which convince user to fill-up all details.

Few basic things before some good examples:

1. Keep the registration form simple and if possible  max to one page.

2. Deliver as many features possible for users without registration, as this will convince users to register.

3. Explain the benefits of registrations upfront to convince why they should spend precious time to fill all details.

4. Grouping of fields as per their logical relation and also educate user about how and where this information will help in future.

5. Keep required fields to minimum, don’t put unnecessary fields which can be ask from user as and when the requirement comes

Twitter

Mint

Facebook

Windowslive

Vox

Box

Netvibes

Compete

Aol

Wordpress

Blogger

Xanga

Hi5

Outright

Deviantart

Basecamp

Yelp

Vimeo

Ning

Metacafe

Mint

Fotki

Myspace

Lulu

Animoto

Wufoo

Geni

Netflix

Ebay

Ballpark

Craiglist

Gowalla

Mobileme

Good Read::

Web Form Design: Filling in the Blanks

Web Form Design Patterns: Sign-Up Forms