I use Mail for Mac OSX Snow Leopard. I have my Gmail linked to non-Gmail email accounts so that I can send mail as them.
Is there a way to use this feature without the web interface (SMTP)?
[Edited, see comments]
In the Mail application you'll want to add a second (or third, etc. if you have more) SMTP account. Then, when composing an email, you can select which account from which you wish to send it. More information: