Mjoy is using advertising within it's interface to support a service which allows it's users to send text messages of any size for free to any mobile phone in the world. Being a mobile web-based service they are also able to, and do, offer a few enhanced services like conversation-sorted messaging ala-Gmail for easy following of to-and-fro sms conversations, multi-source contacts import and mobile video. Registration is simple and quick and the experience not hard to follow, even without reading through the help, and the messages are delivered just as rapidly as normal messages would. The only difference being an additional 'I am texting for free with http://mjoy.com' after your message. Importantly when they receive the message it appears as if you sent it, so when they reply it goes straight back to your phone.
For those who have 3G enabled phones with contracts with cost effective data plans this could change the way you communicate.
Just remember this is not 'free' as you are paying your service provider for the data you view, but this is a whole lot cheaper than sending smses.