VoiceMac is the first Google Voice client for the Mac. Send multiple SMS Messages, send SMS Messages, 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. When you receive a SMS Message or Voicemail, you get notifications via Growl and hear sounds that is customizable.
With SIP support you can place calls from your computer using your favorite SIP service and if you link your Google Voice Number with the SIP service, you can place calls with your Google Voice Number from your computer. With the reverse lookup data, you 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.
VoiceMac's contacts system is one of the best contacts system around, with support for Address Book and Google Contacts. You can search your contacts just like a Search Engine with speed.