Skype

I used this product since it came out but two feature that most people are not aware of are these:
The ability to change your Call ID number to match your mobile etc. is pure genius. I was stuck in the USA at Christmas and needed to operate from my friend home as if I was in the UK.
Other than starting my day at 3am I was able to function as if I was in my office in the UK. I got work to redirect my work mobile number to my Skype In London number. And I had fortunately update my Skype’s Call ID with my work mobile number. This meant that each call from the USA cost nearly nothing but people thought it was me (and answered) and were able to call me. Of coarse I was able to use Google Voice in the USA to bounce incoming calls to any number of phone there (please bring Google Voice to the UK).
The other function is Skype To Go this allows me to assign a local number (normally bundled with mobile minutes) to my Skype account. When I call this number from my phone I’m given the option to speed dial pre selected numbers I have already entered in the web interface or enter the number I want to dial. It’s great to contact my international friends while stuck in the car. I have my phone CallerID added as an authorised number otherwise you have to enter a pin to access this functionality.
If you are in the USA and you get free local calls than this is a great way to call internationally using proper phone and not have to use your computer or mobile app.
www.skype.com

Update - latest version supports dual logins for Win7 so you can have one Skype account for personal use and another for business. Just create a desktop short-cut that has a  /secondary command. Only wish this was simpler for normal users.