Another insanely complex but very useful tool - not for the faint-hearted.
Filing mail for multiple addresses
If you have multiple email addresses that are used by different people for different purposes (ie. one for general use, one for friends, one for important clients, ...) you can use procmail to automatically place incoming messages into different folders for processing.
This will pick up all messages that are sent or copied to email@example.com and file them in the folder mailbox. Just repeat these commands for each of your addresses.
:0: * ^TO_username@example\.net folder
TO_ is a macro defined internally in procmail