If you are sending mail to more than 500 people at once you should take a step back and re-evaluate your life as a spammer.
If you are sending a newsletter to more than 500 people at once who actually WANT to receive your message, then you could look into mailing list services, but they won't be free. Any program you find that does allow you to mail more than 500 people at once might not be tolerated by your ISP, as many these days are very proactive about spamming, mass mailings coming from their SMTP server could get them blacklisted, so you should probably also check your ISP's agreement before doing so. Also you should read up on anti-spam laws and make sure you have an easy unsubscribe method linked in your message.
If you have a large group of people that you would like to keep informed of news and things, you could also consider not sending mass mailings but just set up a blog. There are many free services like blogger.com that make blogging very easy.