iOS: Cryptocat is one of the easiest to use encrypted chat clients out there. Now it's available on the iPhone so you can keep your conversations private while on the go.
Cryptocat for iOS works in much the same way as its desktop counterparts. Create a private chatroom with a unique name (choose something strange and unique if you don't want random people dropping in), share that name with your friend over another form of communication such as a text message, then they'll create a one-time use username and enter the chat room. From there, you can chat away about whatever you want, and Cryptocat won't save any of it after you exit the app.
All the text sent in Cryptocat is encrypted and you can peek at how it works on the Cyrptocat blog. Cryptocat works between devices too, so you can chat with people on desktop, in their browser, or in the iPhone app. I had a few random crashes in the app selecting information about usernames, but overall the experience is similar to the desktop versions.
Cryptocat (Free) [iTunes App Store]