Contact Form 7 for WordPress is one of the best contact form plugins to use on a WordPress blog.
More Info: http://contactform7.com/
One of it’s most powerful features is that it allows you to create multiple contact forms for different purposes. It also supports many features including AJAX submitting, CAPTCHA, Akismet spam filtering, file uploading, etc.
Code Stuff has a great article on how to install, extend and use this plugin at:
http://www.codestuff.com/2009/07/16/153/wordpress-contact-form-7-plugin-great-for-forms.html
Code Stuff also has a Contact Form 7 Backup and Restore plugin