First off: Google first, you'll find many more answers in a LOT less time.
To actually answer your question, it depends. If you just want to send one message, one time then it really is probably just easiest to do it in your regular (web)mail program. If you're sending multiple messages then... If it is not going to be sent to TOO many people, then in Gmail you can just create a "group" of contacts and send mail to them all at once.
If it is to a ton of people you'll have to find something via google. Just google something like "bulk mail program" or "email newsletter program".
To send a bulk email for free, you will have to first determine whether you will send using your own email service or the mass sending program. Secondly, select a mass email platform like vertical response or smart serial mail. Check for more details on http://www.ehow.com/how_5084463_send-mass-email.html.