Chrome/Opera/Safari/Firefox (Beta): It's been a while since we highlighted Gmelius, the add-on that cleans up Gmail's interface. It's grown since then, and now has features to send emails later on a schedule, snooze them, bundle in useful reminders and block email trackers. Gmelius now blocks email tracking attempts, lets you "snooze" emails you want to deal with later to get them out of your inbox and have them reappear when you want to see them and it can schedule drafts to send at a later date and time.
The add-on also makes email templates and canned responses much easier to use, and includes support for rich elements like GIFs and attachments to those canned replies. Finally, Gmelius includes a built-in to-do list manager. There are also other smaller features, like the option to generate return receipts if you tend to get emails from someone who nags you over and over about whether you got their message, and the option to send and label, so you can both reply to a message and organise it at once. It also includes a search plug-in, so you can search your Gmail inbox right from your browser's URL bar.
To be fair, there are other apps, like Boomerang or Google's own Inbox, that offer similar functionality, many of them for free. By comparison, you can sign up for Gmelius for free, but its best and most useful features are behind a $US5 ($7)/month price tag. Even so, if you use a Gmail or Google account for work, or just like all of that control in one place, it's might be worth the dough. It's available for Chrome, Opera and Safari, with a beta version for Firefox available now.