Windows/Mac/Linux: If you use multiple Gmail accounts, you're used to having to open several tabs for everything. Shift alleviates this problem by opening up several accounts in one handy desktop client.
Tagged With email clients
Android/iOS/Web: It seems like everyone has some new product that will change email forever, but there are some good ideas in the mix. AOL's new Alto Dashboard takes a lot of inspiration from apps like Inbox and applies them to all of your email accounts at once.
Mac: Airmail, one of our favourite email apps on Mac, just got a slew of new features today, including smart folders to more easily sort your mail, VIP contacts and notifications and 'send later' email queuing.
iOS: You have countless options for email organisation tools, but EasilyDo Mail is an app that makes automated organisation of some common categories, like travel and newsletters, extremely easy.