Mail supports .Mac, IMAP, POP, and Exchange accounts. As far as I know, it can have as many of each as you want installed. I have, I think... three IMAP accounts and four POP accounts in my Mail.
You just go to Mail menu -> Preferences -> Accounts -> click the plus underneath the list of accounts, and then enter in info about the new account.
All the "server" folders -- inbox, drafts, sent, trash, junk, etc, will have triangles next to them and have sub-boxes for each account.