This is a small list of tools that will enhance your 4chan experience in one way or another. All the programs and websites listed here are to be used at your own risk and are in no way related to this wiki. They're most likely botnets anyway and will get you vanned in no time.
4chan already has lots of features these extensions provide, so you might be fine with not using any.
Browser - Helps you browse 4chan. It's basically an offline version of the catalog with a lot more customization and features. It updates itself, and newer versions can be found by searching /rs/ or Pastebin.
Yotsuba Catalog - Allows easy viewing of all of the threads on a board, with filtering, highlighting, hiding and stickying features. Doesn't have NSFW boards. Can search and filter inside threads instead of just the OP.
Chan Thread Watch - Saves threads from *chan-style boards and checks for updates until the thread dies.
Chan Watch - An iOS app that checks for thread updates. It lets you know if there are new posts, which page it's on, and if the thread has expired.
4charc - Another very useful thread archiver. Backs up any thread in progress on 4chan.
ST4 Image Dumper - Automatically dumps images on a thread.
Tripcode Explorer - Useful for tripfag-related stuff.
WebM for Retards - Easy WebM maker.
4Chan Thread Downloader- Downloads attachments (webm, gif, image) files from 4chan as zipped.
To use these, you need Greasemonkey (Official, possibly slower) or Scriptish (Fork, possibly faster, but not supported and updated anymore) if you are using Firefox. Chrome/Chromium users don't need any, but Tampermonkey could be useful.
4chan X - Adds various features to 4chan which the official extension doesn't already have.
Appchan X - Updated version of Appchan with baked-in 4chan X. Adds styling options.
4chan Style Script - Style script that uses 4chan X too.
Dollchan - An alternative to 4chan X that works with other imageboards too. It's in Russian, but you can switch the language in the gearhead menu.
4chan Media Player - Allows posting sounds in 4chan's boards along with other features.
4sight - Search for threads on 4chan.
ExLinks - Makes ExHentai/E-Hentai links more useful and adds ExHentai/E-Hentai reverse image search (ExSauce).
Linkify Plus - Turn plain text URLs into links.
4chan Linkify - Turn plain text URLs into links. Use this if you have problems when updating threads.
YouTube Link Title - Adds video titles, shows previews and embeds on click.
Moe Archive - /a/'s preferred archive. Full images are limited to some boards and are eventually deleted. Crashed in October 2015 and losing all threads 4 months prior to the crash. Desustorage is now /a/'s preferred archive.
Fireden has the archive.moe backup.
Warosu Archive - /jp/'s preferred archive.
Chanarchive - Users vote and request which threads they want archived. Full images. Dead site.
More archives are available here. The ones above are the most notable archives available.