Blackberry don't make a big deal out of advertising Messenger but for me its right up there alongside Email as the killer Blackberry application.
During my 8 week trip to North America in 2009 I bought a local pay-as-you cell phone as normal. I bought $50 worth of credit with it, which I guessed would last 2-3 weeks at most. With so many people owning Blackberries, more so in Canada, I was able to make great use of the Blackberry Messenger (BBM) feature and I still had some of that $50 credit left after 8 weeks of travelling.
But what is it? Think of it as a combination of a text message and instant messaging.
What are the benefits?
:: it doesn't cost any extra
:: it works via the mobile network and also wifi (should you not have service)
:: you can tell when a message has been delivered and read
:: you can see when the other person is typing a message back to you
:: you can use it to send photos
:: you can record and send short voice messages
:: it works just as well sending a message to someone in the same city as it does to the other side of the world - because of this I was able to keep in touch with people in England from my mobile phone for free when calls and texts would have cost loads.
To connect to another Blackberry user you'll need to swap PIN numbers. To find this look under Options and Status, its normally an 8 digit alphanumeric sequence. Fire up your Messenger application, Invite your contact using their PIN and once they've approved you as a friend you'll be away.