add custom field in billing & shipping magento

how to add add custom fiel开发者_高级运维d in billing & shipping magento i want to add Residential & Commercial to be choose from radio button during billing and shipping check out page. i am using magento 1.4.2.0

please help i want code or some free extension urgently

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Here is a wiki which describes what to do and has code samples:
http://www.magentocommerce.com/wiki/5_-_modules_and_development/customers_and_accounts/registration_fields