If you are using a Wordpress theme which doesn’t support users subscribing to comments (crazy I know) you can add the plugin Subscribe to comments which is currently at version 2.1

A summary of the features this plugin provides:

• Rudimentary security
• Recognition of subscription status
• Themeable subscription manager
• Ability to let users change notification e-mail address
• Easy integration into comments form
• Subscription without leaving a comment
• Customization
• Internationalization

Installation is as easy as you’d expect from a Wordpress plugin. Simply upload 1 php file into the plugins directory, then “activate” the plugin in the admin area.

Then all you need to do is add a line of code to your theme comments.php where you want the checkbox to go and you’re ready!

Read more and download the plugin here.