Chrome: Your inbox gets distracting (unless you keep it clean), and sometimes you just want to send an email without seeing everything else requiring a response. Open Compose lets you click a button and compose a message in Gmail without distractions.
All you have to do is install the extension, then click the little Gmail button in your toolbar to get a new message. It serves a very specific purpose, but it's a nice little shortcut to have if you compose emails frequently.
Those of you not on Chrome -- or who would just prefer a link -- can bookmark this URL to go straight to the compose window too.