G-Mail is the most reliable open source email app on the world. I have many domains and personal email IDs but I still use G-Mail for my official/personal use. I have more than 8 G-Mail ids , so I have installed 5 Web browsers to access different email IDs in different browsers as multiple G-Mail log-in was not possible easily. But now after Google redesigned the G-Mail and Google account interface, They have added lot of new features to enrich customer experience. Multiple accounts usage is one of those features.
By using this option you can access more than one Gmail accounts at a time. You can login as many number of gmail accounts as you wish. You can log out a particular account if needed or you can also log out all accounts at once.