Wmail is an open-source Gmail and Google Inbox client for the desktop (Windows, OS X, Linux).
The program works just as you'd expect. Add your various accounts, as many as you need, then browse, compose or fetch new emails as required.
You could do this with a browser, of course, but Wmail gives you better support for drag and drop and keyboard shortcuts, as well as providing native desktop notifications when a new message arrives.
Performance is better, too - no multiple tabs or waiting for pages to load - and because Wmail is based on Chromium you can be sure it'll render all of your messages correctly.
Early Wmail builds had plenty of issues, particularly with notifications, but many of these have been addressed. If you had problems with a pre-May 2016 version, give it another try.