While iMessage has its own native Mac app, you also have the ability to quickly send an iMessage via the Contacts app if you're running OS X Mountain Lion. Whether you're looking for a specific number or not done looking through your contacts just yet, here's the easiest way to get a message sent without interrupting what you're doing within the Contacts app.
- Launch the Contacts app on your Mac.
- Search for or scroll to the contact that you'd like to send an iMessage to.
- Click on the label for the number or email you'd like to send the iMessage to.
- Now click on Send Message.
- An iMessage window will now appear allowing you to quickly send a message to the contact of your choice.
You'll notice that the send button is greyed out if that address is not an iMessage address. In this case, you'll have to choose a different address for that contact.