The Mail app in Lion provides a number flag colors that let you not only flag items for later review or follow-up, but categorize as well.
But just flagging, even by color, still means you have to go back through message lists to find those flagged messages and if you have 7 email addresses like I do, that can get to be a drag.
You can solve that problem by creating smart mail boxes that will sort and save your messages according to flag color.
Select Mail > Mailbox > New Smart Mailbox. From the first menu select “Message has Flag”. From the second select your flag color of choice. Name the mailbox and hit OK. Any messages you have market with a flag of that color will immediately be listed in that Smart Mailbox.
You will have to repeat this process for each flag color because of the Mailbox naming process.