skype allows you to make free phone calls to anyone else using skype. this is a free download. also, if you want to call a regular phoneline you can use skypeout which is not free but is a cheap way to make landline calls.
the skype program also encrypts all calls and then decrypts them at the reciving end. it also allows file transfers between skype users. skype is available for mac, linux, pocket pc, and windows. mac users can download it from the apple skype site