Windows/Mac/Linux: Although there are a bunch of ways to back up your Gmail, including really easy to use Windows app MailStore, cross-platform program Gmvault takes the cake for being feature-rich and highly customisable.
Gmvault not only backs up your entire Gmail account to disk, you can set it to run regularly to always keep your backup up to date (it's fast too; in the quick sync mode, it only updates the latest emails). You can restrict your backup to a set of certain emails based on a search request or date range and also encrypt the saved emails, which are stored in a Gmvault repository.
Gmvault's most unique feature is its ability to restore your mailbox, complete with labels and other attributes, to a different Gmail account. This makes the app useful also if you ever want to move to a different Gmail handle.
It's a console-based application, so you don't get a fancy graphical user interface (GUI), but the program is still easy to use. Type in "sync [your email address]" to start the backup and "restore [email address to restore to]" to restore the emails. If you need help, type in
gmvault -- help or consult Gmvault's quick start or in depth guides.
The simple app makes regularly backing up your Gmail account or moving to a new Gmail account a cinch.