You can also go old-school IM (ICQ/AIM/etc). Or you can go for a multi-protocol app (like Fring, eBuddy, Trillian, Meebo) and let everyone choose how they want to communicate. Most of the multi-protocol clients for the Android OS do support GTalk (Google Chat).
Personally I like a messaging program called Kik. It's great if you know other people who use it. It's a very simple messaging program that lets you send/receive messages almost instantly like you would get them on AIM or MSN on the computer.