The most popular messaging app by far is WhatsApp Messenger, which has just crossed the "one-billion-messages-in-a-single-day" milestone. However, it only allows you to send free text messages. Also, do note that the app is only free for the first year and costs S$1.99 per year subsequently.

Two other popular apps are Line and Viber. Like WhatsApp, these free apps allow users to send text messages as well as make free voice calls, by using either 3G or Wi-Fi. Group messaging and file transfer options are also available.