This hack cuts the bandwidth spammers steal when they try to hit your blog if they are in your antispam table. Easily installed, and the biggest (first) part can/will survive upgrades.
This hack provides one of the most requested features the forums have ever seen. If you apply this hack and have open registration, then a person can register at your site and have a blog automatically created for them, including permission to post on it. As EdB says, use this at your own risk.
Add avatars for authors.
This hack creates a stripped-down version of the update script, which can be triggered using a cronjob so that antispam updates are done automatically at regular intervals.
A hack that allows you to quickly add images and links from Amazon Web Services. It can be used to make a "Now Reading" sidebar. Plug in your own Amazon Associates ID and when people click through and buy the book, you get a few cents.