QQ from Tencent is one of the most popular instant messaging programs in Asia. Originating in China, it is widely used among the Chinese communities in most countries, with more than 100 million accounts active. The iPhone and iPad app can be hard to locate (it does not exist on U.S. iTunes) but is available through numerous sites. QQ installs easily although the interface presents in Chinese characters only.
To use QQ as a messaging platform, you need an account. Once online you can see friends who are also online, and have the ability to conduct multiple chats simultaneously as well as transfer files. There are some neat features as well, such as the ability to exchange locations using the iDevice's location services (handy for meeting up with other QQ members), offline message retrieval, and panes to split the screen into separate sections.
If you can't read Traditional or Simplified Chinese, QQ won't be useful to you until they localize it (no time frame available yet). For those who do understand Chinese, QQ is almost a mandatory app for iDevices because of its ubiquitous installation throughout the Chinese community.