WhatsApp messenger has been updated to work on iOS 4 and iOS 5, after the recent 2.11.6 update had a bug that instantly crashed the app for those users.

WhatsApp Messenger is a smartphone messenger available for iPhone and other smartphones. WhatsApp uses your 3G or WiFi (when available) to message with friends and family. Switch from SMS to WhatsApp to send and receive messages, pictures, audio notes, and video messages

whatsapp is FREE for now too