I've been using Mike's subscribe plugin on a site I host for some time now. It works well enough, but it always kind of irked me that it uses a text file to store the list of subscribers, instead of the database. It also irks me that there's no real verification to the subscription or unsubscription process.
So I reworked it! Here is subscribe2, released as 1.0 beta.
To use this, you need to create a link somewhere on your blog to the subscribe.php file. Here users can elect to sign up for updates when you post new entries. Each (un)subscription request sends a confirmation email verifying the legitimacy of the request.
The subscribe2.php plugin file contains an administrative interface to install the database table, and manage your subscription list. You can add subscribers in bulk, as well as individually enable, disable, and delete subscribers. Plus you can send an email to all your subscribers at once.
With WordPress 1.5 just around the corner, the obvious next step for this plugin is to make it integrate into the WordPress admin panel.
There are probably some rough edges here. Please let me know if you find any bugs, but I accept no responsibility if this plugin kills your dog or eats your archives.