Gmail "On Behalf Of" Feature in Mail.app
Solution 1:
Under Accounts in Preferences, there's a field there called "Email Address:". You can put a comma-separated list of addresses in there, like this:
Now your "Compose" window will have a "From" popup menu that lets you choose your sending address:
AFAIK, this works for pretty much any mail account type, as long as your mail provider allows you to muck around with the From: header.