My favourite so far has been Google Talk. To use google talk you will need to sign up for a Google G-mail account and have your friends and family and co-workers sign up too.
With Google Talk you can instant message without incurring Blackberry text message fees. Your Blackberry rings like the telephone when someone is trying to contact you. When you wake your Blackberry you can just start typing. It's that easy.
Next time I'll talk about Google Calendar and Google Sync.