SplitPersonality is an Add-In for Outlook 2003 and Outlook 2007 that allows simple multiple outgoing email personalities from the same Outlook account.  Designed for people who wear many hats, this program simplifies the process of sending emails under different addresses, doesn't need any complicated mail server set, allows for custom signatures, etc.

Full details at http://www.outlooksplitpersonality.com

Enjoy!