WhatsApp Messenger is a smartphone messenger available for Nokia, iPhone, Blackberry, and Android phones

WhatsApp uses your 3G or WiFi (when available) to message with friends and family.
Switch from SMS to WhatsApp to send and receive messages at no cost.
Free for the first year!

Suitable for anyone over 9 years old.
May contain
Infrequent and mild occurrences of fantasy violence
Mild physical intimacy such as kissing
Need for downloads or online communication between people

Note:
WhatsApp is giving its service free for 90 days to this new Beta release for Symbian devices.
During, or after free trial period you can purchase service for $1.99 USD/year at http://www.whatsapp.com/

Supported S40 phones:
Nokia C3-00 · Nokia C3-01 · Nokia X2-01 · Nokia X3-02 · Nokia X2-00 Nokia Asha: 201 · 205 Chat Edition
· 206 Single Sim · 208 · 210 · 300 · 301 · 302 · 303 · 305 · 306 · 308 · 309 · 310 · 311 · 515 · 500 · 501 · 502 · 503 · 230