No need for a hack. Geeklog supports this feature out-of-the-box, you just need to enable it in geeklog/config.php.
Sorry, I don't have access to the files currently, but if you search for 'emailstories' in config.php, you should find it.
When you turn it on in config.php, every user will have an additional section in their preferences page. They can select for which topics they want to receive new stories by e-mail.
The only trick is you need to set up a cron job to send the e-mail digest out once a day. If your server doesn't offer cron, google for pseudo-cron and install that.
Hope this helps.
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