eBuddy XMS is a free messaging app for Nokia Series 40 phones that allows you to exchange text messages and photos via your data connection.
To set up messaging with eBuddy XMS, you need to enter your phone number, then an activation code (sent to you via SMS). You'll be issued with a PIN number once you've entered the code. You can then chat with anyone on eBuddy XMS, adding them as contacts by entering their phone number or PIN number. There's an option to scan your phone book or Facebook friends for any contacts who are using eBuddy XMS.
In terms of features, eBuddy XMS lets you send photos as well as text messages. Unlike WhatsApp, there's no option to record and send video or audio files, however.
Providing you can convince your friends to switch from WhatsApp, eBuddy XMS makes for a simple, free messaging app.