I’ve met some Mac users who want to be able to send iMessages without having the actual Messages app and all its tempting distractions open.
Fortunately, there’s an easy way to do this from the Social widget in the Today page. To open that page, two-finger swipe from your trackpad’s right edge, or click its icon in the far right of the menu bar. Make sure it’s on the Today tab, not Notifications.
If you don’t have the Social widget enabled, click Edit at the bottom of Today and add it. Then, use the fourth button on the right to open a simple iMessage compose box. You can’t
send images (see below) read incoming messages, but it’s an easy way to quickly fire off a message and get back to what you’re doing.
Update: Turns out you can paste images and files from your clipboard, but only with ⌘-C and ⌘-V; no drag-and-drop. [thanks Craig Morgan!]