Mailchimp will allow you to send 10k emails monthly for free but they have to be your clients already. Is guess I should have asked you first... Is this to find new clients or touch base with your existing clients
There are a lot of different email marketing tools for launching email campaigns. For example: Mailchimp, eSputnik, Activecampaign, Mailerlite etc. You can try all these services for free and choose the most convenient and flexible for your needs.
If you're looking for some useful tips on how to send emails the right way, so visit Hubspot blog, Constant Contact or eSputnik. You will find a lot of articles about latest email marketing trends.