1.Beejive IM (the full version is priced at $ 9.99, free trial version supports only one account)
BeejiveIM for GTalk lets you access Google Talk on the go, with the proven reliability of BeejiveIM. Now you can chat with your GoogleTalk buddies anywhere you go!
2. EBuddy (free)
EBuddy for iPhone and iPod Touch is one of the most popular IM apps in the world, with more than 5 million downloads.. It support chat on AIM, MSN / Windows Live Messenger, Yahoo, Facebook Chat, GTalk, MySpace, Hyves and ICQ, that you do not need to install a bunch of software.
3.IM + (regular edition: Free; Professional Edition: $ 9.99)
IM + has the best interface among the five apps we recommend. With it, we can chat on Skype, Facebook, Google Talk, MSN, Yahoo!, ICQ, and more. IM+ features free texting, Off-the-Record messaging as in-app purchase, group chats, message history, multimedia sending and much more.
The free version contains advertising, but you can remove them by spend $ 3.99. Professional Edition is entirely ad-free, and also contains more features. Both versions can be used on the iPad, you can sync it via iCloud.
With it, you can talk to your friends on any IM network! Imo supports Facebook Chat, Google Talk, Skype, MSN, ICQ/AIM, Yahoo, Jabber, Hyves, VKontakte, MySpace, and Steam. Also, it can send short voice messages
5. Verbs (0.99 U.S. dollars)
Verbs is a very flexible chat application, it has some features not seen in other app. It can sync your Google Chat contacts on the iPhone; it also allows you to save the link address.