Features

Accounts

Account Switcher
Store username/password pairs and switch accounts instantly while browsing Reddit!

Show Comment Karma
Shows your comment karma next to your link karma.

Username Hider
Hides your real username when you're logged in to reddit.

Comments

Comment Navigator
Provides a comment navigation tool to easily find comments by OP, mod, etc.

Hide All Child Comments
Allows you to hide all comments except for replies to the OP for easier reading.

Live Comment Preview
Provides a live preview of comments, as well as shortcuts for easier markdown.

New Comment Count
Shows how many new comments there are since your last visit.

Save Comments
Allows you to save comments locally, to easily view them in the future!

Show Parent on Hover
Shows parent comment when hovering over the "parent" link of a comment.

Filters

filteReddit
Filter out NSFW content, or links by keyword, domain (use User Tagger to ignore by user) or subreddit (for /r/all).

Spam Button
Adds a Spam button to posts for easy reporting.

Subreddit Tagger
Adds tags to posts on subreddits (i.e. [SMT] on SomebodyMakeThis when the user leaves it out).

User Interface

betteReddit
Adds a number of interface enhancements to Reddit, such as a Subreddit manager, "full comments" links, and more.

Inline Image Viewer
Opens images inline in your browser with the click of a button.

Keyboard Navigation
Upvote, downvote, hide posts and most other major reddit actions using only the keyboard. Fully remappable!

Never Ending Reddit
Gives you a never ending stream of reddit goodness- Just keep scrolling down and content loads automatically!

RES Dashboard
A fully customizable dashboard that displays content from subreddits, message inbox, user pages and more!

Single Click Opener
Adds an [l+c] link that opens a link and the comments page in new tabs for you in one click.

Style Tweaks
Provides a number of style tweaks to the Reddit interface such as rounded comment boxes, colorblind friendliness and moving the username bar.

Subreddit Manager
Allows you to customize the top bar with your own subreddit shortcuts, including dropdown menus of multi-reddits and more.

Users

User Highligher
Highlights certain users in comment threads: OP, Admin, Friends, Mod - contributed by MrDerk.

User Tagger
Adds a great deal of customization around users - tagging them, ignoring them, and more.