SandboxGeneral has it right, and I can also recommend Adium. One thing worth noting is that Messages works with Apple's iMessage protocol. If you have an iPhone running iOS 6 and you update your computer to OS X 10.8.2, your computer can receive texts sent to your phone number through iMessage, and can reply as well. Note that this does not apply to standard SMS texts: you can only message people who are using devices running iOS 5 or higher, your message will not go through if their phone isn't connected to a data network (whether wifi or cellular), and your computer won't see messages that are sent via SMS (messages from people not using iOS-devices, using an iPhone with iMessage disabled, or if the iOS device switches over to SMS due to lack of a return receipt).
That probably sounds more complicated than it really is. Suffice it to say, it's a pretty neat feature. Messages sent and received on Messages will show up on the iPhone as if they were sent and received there, and vice versa. If you have an iPad or iPod Touch, upgrading it to iOS 6 allows it to also participate in the same manner.
