Nifty: Postfix Enabler turns any Mac into its own SMTP server, useful for laptop users finding themselves god-knows-where without an SMTP connection. Also includes a full POP/IMAP server, if you swing that way. Postfix is already built into OS X, but unless you’re running Server, you’ll have to jump through a few hoops to get it working. For $10, Postfix Enabler makes it happen in less than a minute, cleanly, con GUI.
Of course if your ISP requires mail to be routed through them, it’s not going to help a great deal – you’ll just have to configure Postfix to authenticate to the ISP anyway. But by telling your mail client to send outbound on localhost, sending at least feels buttery fast.