Skype

Skype is your modern PC phone. If you've been using built-in PC phones like those found in Instant Messengers (IMs), you may find Skype useful also. If you are using a stand-alone PC phone like PC-Telephone, maybe Skype is your better choice. Skype, in fact, is an IM itself (if you prefer to call it so), only with more functions of internet telephony. It is said to have clearer voice. I believe so, but I didn't actually compare its voice quality with other PC phones. One good feature I like about Skype is its 5-person conference call. It lets five people talk to each other at the same time! Skype is also cross-platform and multilingual! All PC-to-PC calls are free and PC-to-phone calls are relatively cheap. Signup is required to use its service. Download size is about 10MB. See the screenshots!