Extension Manager

Edit the alias list
Provides several automatic tagging functions
Adds autocomplete to search & tagging.
Turns BBCode into HTML
Provides query and selection-based bulk action support
Show a "down for maintenance" page
Allow users to favorite images
Bring a specific image to the users' attentions
Allow users to upload archives (zip, etc)
Handle JPEG, PNG, GIF, WEBP, etc files
Ban IP addresses
Show various forms of link to each image, for copy & paste
Allow users to make comments on images
Show a list of uploaded posts
Allow users to rate images "safe", "questionable" or "explicit"
Allow users to send messages to eachother
Allows displaying a page with random posts
Do things with a random post
Regenerate a thumbnail image
Report posts as dupes/illegal/etc
This extension allows admins to resize images.
Allows admins to rotate images.
A simple wiki, for those who don't want the hugeness of mediawiki
Keep a record of source changes, and allows you to revert changes.
Show various bits of system information
Allow images to have tags assigned to them
Keep a record of tag changes, and allows you to revert changes.
Recount / Rename / Etc
Make some users only visible to themselves
Send PMs in response to certain events (eg post deletion)