Liberal use of the "lolwut" tag is recommended.

Extension Manager

Admin Controls Various things to make admins' lives easier
Alias Editor Edit the alias list
BBCode Turns BBCode into HTML
Browser Search Allows the user to add a browser 'plugin' to search the site with real-time suggestions
Bulk Add Bulk add server-side images
Comment Word Ban For stopping spam and other comment abuse
Danbooru Client API Allow Danbooru apps like Danbooru Uploader for Firefox to communicate with Shimmie
Downtime Show a "down for maintenance" page
Favorites Allow users to favorite images
Featured Image Bring a specific image to the users' attentions
Handle Flash Handle Flash files
Handle ICO Handle windows icons
Handle MP3 Handle MP3 files
Handle Pixel Handle JPEG, PNG, GIF, etc files
Handle SVG Handle SVG files
IP Ban Ban IP addresses
Image Comments Allow users to make comments on images
Image Hash Ban Ban images based on their hash
Image List Show a list of uploaded images
Image Ratings Allow users to rate images "safe", "questionable" or "explicit"
Image Scores (Numeric) Allow users to score images
Image Viewer Allows users to see uploaded images
Image Zoom Scales down too-large images using browser based scaling
Link to Image Show various forms of link to each image, for copy & paste
News Show a short amount of text in a block on the post list
Pools System Allow users to create groups of images
Popular Images Keeps track of image views and the most popular images of the day.
Private Messaging Allow users to send messages to eachother
QR Codes Shows a QR Code for downloading an image to cell phones.
RSS for Comments Self explanatory
RSS for Images Self explanatory
Random Image Do things with a random image
Random Tip Show a random line of text in the subheader space
Regen Thumb Regenerate a thumbnail image
Report Images Report images as dupes/illegal/etc
Tag Editor Allow images to have tags assigned to them
Tag History Keep a record of tag changes
Tag List Show the tags in various ways
Uploader Allows people to upload files to the website
User Management Allows people to sign up to the website
Word Filter Simple search and replace
XML Sitemap Adds sitemap.xml on request.