The number of tools and resources that Google provides is endless, and they add new services on a daily basis. Google Voice Calling is Something I Use in my personal and professional life.
At home, I use Google Voice Calling when our landline is tied up, or if I'm multi-tasking and just want to make a call from the computer. It's the easiest service to register for and set up, and very handy to use.
At work, I use Google Voice if I'm somewhere without a landline or my mobile phone. You can also text from Google Voice which is real handy. We're slowly converting my district to Google Apps and I'm hoping we incorporate mail into the Google App equation.....which will allow me to call from my work number, rather than having to use my personal gmail account.
Google just announced here that you can now make two calls at one time, just put the other call on hold while you dial on your second Google Phone.....very cool!