MailFred let's you temporarily archive and set reminders for GMail. Mails will come back to your inbox when you need them. It works in Google Chrome and Firefox with GMail and Google Apps For Your Domain and looks like this:
MailFred is in no way affiliated with Google, Inc.
The source code for our extension is open in case you want to contribute (which we would really appreciate).
All of the components of the extension are open source as well, for example we use eventr, which is a Javascript event simulator to fire events on the buttons in GMail. We use GMailr, which is a Javascript API for GMail to find message IDs and whether you are in a conversation or threaded view and gmailui to generate buttons and overlays which look exactly like native GMail.
Our website has more information about our free service and the extension in an installable form can be found here for Google Chrome and a here for Firefox beta!.
We would actually love to have new translations. So far we only have en and de - if you'd like to contribute, you can either do so here, by translating the according JSON file into your language or if you are not a developer, we are more than happy to give you access to our project on CrowdIn.