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.
Shift gives you separate tabs to access Gmail, Calendar and Google Drive within a single window. If you want to switch to a different account, just click the profile tab on the left side of the window. Shift is free for two accounts. If you want more than that, it's $US20 ($28)/year for unlimited accounts, which also includes support for Outlook.com accounts.