"free" messaging pc to cellphone?

:D I know nothing about IM or AIM which are located in my sony cellphone (tm506). I am not even sure :eek: that this is the right place to ask. I see websites which claim to allow free messaging (via email) from pc to cellphone. They use :confused: phone [email protected] (for example) and state that this is a free message. My question is, how? Am I not charged by the bloodsuckers at tmobile for an outgoing message and if I get a reply, a charge for an incoming message? :p
:eek:All I want to do is to send messages from my pc to cellphones. I don' t
understand what exactly is "free".
Is there another way I could do this that might be more accessible from my xp o/s? Thanks in advance.;)