Google provides a large number of easy-to-follow guides on how to setup Microsoft Outlook, etc. with your Google Apps email.

• Microsoft Outlook 2007
  • http://mail.google.com/support/bin/answer.py?answer=86374
    
• Microsoft Outlook 2003
  • http://mail.google.com/support/bin/answer.py?answer=75291
    
• Apple Mail 3.0
  • http://mail.google.com/support/bin/answer.py?answer=86393
    
• Thunderbird 2.0
  • http://mail.google.com/support/bin/answer.py?answer=86399
    
• Windows Mail
  • http://mail.google.com/support/bin/answer.py?answer=86383

If your email client isn't on the list above, check Google's full list of guides on this topic at:

• http://mail.google.com/support/bin/answer.py?hl=en&ctx=mail&answer=12103