I wasn't sure where to put this because it's not entirely a feature request although I'd like to see it implemented in the site itself. It's a script I put together that's usable as a bookmarklet (tested in Chrome only) that allows users to upvote and downvote straight from the post listing. Because of limitations with the site's API it's unable to show upvotes or downvotes that were made before the page was loaded. However, it sends an additional vote automatically if an existing vote is removed on first click to make sure that the user doesn't mistakenly remove a vote they already made.
My hope is that this will serve as inspiration for an actual site feature. For now, Chrome users can take advantage of this partial implementation. To use it, copy the following script from pastebin to the URL of a new bookmark in Chrome. Now, go to the post listing (/post/index) of e621 and click your new bookmark. Happy voting!
Updated by Rainbow Dash