Windows only: One of the problems with using multiple accounts in Microsoft Outlook is that each account has a separate inbox folder, so the Addictive Tips blog walks you through the process of using rules to create a unified inbox.
The idea is very simple, just create a new folder to store your unified inbox, and then create rules from each account to move the messages into the single inbox when the messages arrive. It's a simple tip, but could really come in handy, especially if you have a primary email account and a number of secondary accounts that you don't check as often — you can simply make rules to move the messages into your single inbox. The guide is written for Outlook 2010, but you should be able to use a similar technique in previous versions of Outlook.
Set Up Single Inbox Folder For All Accounts In Outlook 2010 [Addictive Tips]