If you are connected to the inernet via WLAN at home for example - then its free.
Its basically just the same as using your computer to do it. N information will go over the cellphone carrier så you don't need to pay for that service.
cost money or are they free? I just got the blackberry. I like the aim application but the ims are really bringing up my phone bill. If i use wifi and use it, will it be free?