Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
i've tried both im+ lite and nimbuzz. Nimbuzz is great, the only downside is that you have to sign up for a nimbuzz acount (really simple though, and can be done rihgt inside the app)

im+ has simple text chatting, that's it, nimbuzz supports smileys, personal message, and also adiuo chat (if you have a microphone). Nimbuzz also supports skype, which most of these apps don't, and it has contact search, so if you have a lot of contacts, definately go for numbuzz;)
 
The best one that i have ever used is Fring it allows you to talk on many other different services. Although if you want something really simple that can go in landscape mode for a larger keyboard i would go with im+
 
My son has used Fring and IM+ lite and has now taken Fring off his touch.

IM+ lite has a landscape mode which he prefers. Fring requires you to set up a Fring account but IM+ lite just uses your existing information.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.