Android/iOS/Blackberry: There are lots of apps that can send "free" text messages, but most require your recipient to also have the same app. JaxtrSMS is a free service from Sabeer Bhatia, founder of Hotmail, that can send free texts to any mobile number in the world, and display it to the recipient as coming from your own phone number.
The app seems to work fine in tests on Lifehacker AU's BlackBerry; replies you receive seem to appear in the app rather than the native message client, but you can send to any number. If you've got plenty of data credit but not so much actual funding for texting on your smartphone, it could be useful. It's also a potentially appealing option for international texts, which are rarely free even on plans which offer unlimited texting.
JaxtrSMS is a free app for Android, iOS, Blackberry, J2ME, Symbian and Nokia phones. Use the link below to find the download link for your device.