Whatsapp is probably the biggest and best known of the chat apps, but yes, there are several alternative devices. And a lot of people would like to change now that Facebook has bought Whatsapp. One example of these is the ChatOn. This is made by Samsung and works with any other Samsung phone. It also works with Blackberry Torch, iOS and others.
One very popular alternative is Line. You can do everything with this that you can do with Whatsapp, even including sending videos and audioconferencing. Line is growing hugely with over 350 million users so far. It's owned by the Korean company Naver but based in Japan.
Another popular alternative is Viber. It's free, like the others, and at present is only available for use with smartphones but will soon be usable with a Blackberry handset too.
You can find further examples of alternative apps here. Many more are available online.