
Skype
Most popular VoIP software, good for PC to PC, support conference call between several PCs. I did experience frequent disconnection when using conference calls. PC to Phone is decent, with a little bit of background noise. You need to pay for Phone to PC, get a temporary phone number, but only on a quarterly(3 months) basis. It also support PocketPC, Palm, and even Linux.

Stanaphone
I have recommended it to soo many people. It is a New York area based VoIP company. Register free for a New York phone number (no "212" of course). Free to accept phone calls, and free PC to PC. My Dad, with little computer knowledge, installed the client software in China, so that he can make "domestic" phone calls to my Cell in U.S. The bad thing is that it becomes hard to connect in the weekends. The sound quality also degenerated.
Vonage
Large VoIP company, recommended by friends.
Mediaring
Easy to use, reliable. It is the only method I found working, hours after 9/11 when the I tried to call China. Sucks for the fact that you will have to recharge every 6 months, or they will void your balance.
Net2phone
This is a old school memory, they provide free phone calls to U.S. Don't know the current situations of this company.
Seawolf (Hey, Nature, I will collect ads fee from you :) )
Company backuped by Stony Brook Alumni.
IMs with chatting capability.
Yahoo messenger
MSN messenger
Google Talk

(Just got to know Google has their own IM with voice chatting capability.)And etc ...