whatsapp messenger
what makes it different from AIM and other messengers is that login is not required, your unique id is your phone number, and it's done over data (either 3g or wifi) so unlimited free texting across carriers and even countries... i can text my friends in south korea who are using whatsapp also for free.
bad thing is that it doesn't function without wifi or 3g on, so if u have it turned off or are in an area without either then u won't receive or send texts thru the messenger.
another great thing about it is the speed and responsiveness, sending and receiving texts occur almost instantly when compared to the lengthy time it takes to send a SMS over the phone network.
my friends who use iPhone and Blackberries are all on whatsapp, so it's basically the hip new messenger to be on until the next best thing comes out. good news is that it's free for android users while iphone and bb users have to buy the app lol.