Google recently released a new service called Inbox. It provides some of the functions previously available through third-party apps such as Boomerang, but it looks like it will also do more than these apps (which only provide limited use; you have to pay them for unlimited use). Read about Inbox at Lifehacker and from and the Gmail blog.
Inbox is currently invite only, though you can send google an email at firstname.lastname@example.org requesting an invite from them when they make more available. I’ve already sent my request to Google, but if someone already has invites, I would be very happy if you’d send one my way!
Update (evening of 10/23): I’ve been using Inbox today and see great potential for it, though I will need to use it longer to develop a conclusive opinion.
Update (2015/03/04): I still think Inbox has a pleasing user interface, but I’ve found it less efficient than regular Gmail. I haven’t used Inbox for months.