The Reddit Enhancement Suite (RES) is a browser extension for Firefox, Chrome, Safari and Opera that gives you customizable modules and keyboard shortcuts to make your Reddit experience an efficient one.
If you spend large chunks of your day or evening on Reddit, then the RES is worth checking out if only for the keyboard shortcuts. Below are, by my estimation, the 10 most useful shortcuts the RES offers.
1. Show or hide all images
You can click the View Images tab at the top of a subreddit to toggle between showing and hiding all of the images of a thread. But the top menu bar isn't static, so it's easier to hit Shift-X to show and hide all images.
2. Move to the next or previous post
To move to the next post, hit the J key. To move back to the previous post, hit the K key.
3. Load the next page
You can jump to the bottom of current page of the thread you are viewing and load the next page by hitting Shift-J.
4. Return to the top
You can quickly go down a wormhole on Reddit. To return to the start of a thread, hit Shift-K.
5. Follow a link
Instead of clicking on a link in a comment to open it, just hit the Enter or Return key. To close the link and return to its thread, hit the Backspace or Delete key.
6. Open comments
To open open a link along with its comments, hit the C key. To open the link and comments in a new tab, hit Shift-C.
7. Open a link and its comments
To open a link in one tab and its comments in another tab, hit the L key. Shift-L does the same thing but keeps the new tabs in the background.
8. Meet the parent
There are comments upon comments upon comments in Reddit. To move up a level in the comment hierarchy, hit the P key and you'll move up to the parent of whichever comment you are viewing. Keep hitting P and you'll eventually return to the first comment. Meanwhile, hit Shift-P to see a small pop-up window that shows the parent comments for the comment you are currently viewing.
9. Up- or down-vote
If you like a comment, you can up-vote it by hitting the A key. Conversely, you can down-vote a comment you don't appreciate with the Z key.
10. See all shortcuts
To see a handy cheat sheet of all of the above shortcuts and many more, hit Shift-/ to call up the master RES list of shortcuts.