I want to send email to the users of my site, individually or to all of them at once.
Is there any available module?
I am using Drupal 6.
Use this Mass Contact module it will match your requirement.
The Mass Contact module allows anyone with permission to send a single message to multiple users of a site, using its roles functionality.
The Advanced User Module allows users who have "administer users" permission to send mail to selected groups of users, and that includes selection by role.