Google talk
Google talk is great if you want a simple, free, pc2pc voice communications program. Both ends need a microphone and speakers. Most new laptops have a mic built-in, but the quality is sometimes not that great. External microphones are typically better and range in price from really cheap to really expensive. I have a $16 Altec Lansing headset that I use occasionally. I rarely do voice over internets calls, but this is a nice thing to have if you are low on cell minutes or are already sitting in front of your rig paying bills, uploading pictures to vistaprint, or playing SNES online with your joes.