Yahoo PC to Phone Calls 1 penny per minute:

Use Phone Out to call landlines and mobile phones with great prices and clarity. With low rates and great sound quality, you can talk for hours and still save on your phone bill.

    http://voice.yahoo.jajah.com/home/rates.castle#section20

Buy credit in $10 or $25 incremements. With $10 talk for 1,000 minutes or 16.7 hours. Like Skype, 1-800 numbers are free, so you can get free calls. See this post.

    http://blog.pcsitter.info/2009/10/19/skype-free-calls.aspx
 
Like Skype and MSN, Yahoo Messenger pc-to-pc is free. Google Talk is another that is PC to PC free, and it also leaves voice mails.