In osx mountain lion you can send imessage messages (or google chat / facebook chat if thats setup in your messages app) directly from the addressbook / contacts app
- open addressbook / contacts
- find the contact you want to message
- click on the ‘mobile’ or ‘phone’ or ’email’ tag next to their phone number / IM address
- a context menu will appear with send email / send facetime /send message
- if you pick message then a popup box will appear
- type your message and click send
- if you pick email from the menu then it will let you compose a new email, and if you pick facetime then a facetime window will open