There is also a program called Palringo.
www.palringo.com . Pretty decent program. It can be used with many other clients also like MSN, AIM, Yahoo Messenger, ICQ, and even Google chat. Whats also kinda cool about it is in the Palringo chat portion, you can use a Push to Talk feature, also share pictures and other files, and create your own groups. There is also a PC version as well so u can still chat with your same contacts from a main computer.
Just try it out, its free, and I liked it better than Octrotalk.