VoiceMac is an application that helps you send and receive SMS Messages, place calls, look at your call history, receive voicemail, reverse lookup a phone number, and search your contact list in one easy interface.
With SIP support you can place calls from your computer using your favorite SIP service and with certain settings can see who is calling you, even if their number isn't in your contacts list.
With themes, you can customize your SMS Messages look anyway you like with simple HTML or by downloading a theme.