Everybody knows how to make Google Voice work, but what about using the Google Voice to get free home phone line that you can use (without any 3rd party software) to make calls from your N900 too (from anywhere in the world)?

So, what do you need:
1. Google voice account
2. Obi110 ATA ($49.99 on Amazon)

Configure your obi110 ATA with the Google Voice account you have (very easy process, explained on obi's website). Connect your home internet and your home phone to the ATA and you have free in/out calling for US and Canada for your home phone.

To use it from your Nokia N900, just setup a Google talk account and call the Google talk account that it is used for Google voice (they must know each other and the N900 account should be a "trusted" account on obi). Now call the Google Voice account from your N900, the AA (Auto Attendant) will answer.
Once you reach the obi AA, you can press 1 to "continue this call", (to ring the OBi phone),
Or press 2 to "make a new call", (i.e. dial a new number, let's say 1-503-xxx-xxxx or 011-40-xxxx .. ). Make sure to press the # key at the end of the dialing.

That's about it :-)

I have this setup at home, and I'm very happy.