One of the big changes in Gingerbread is the inclusion of a full Internet calling stack in the Android operating system. This means that you can make and receive voice calls over 3G (or 4G or any random number of G's) and, more important, WiFi. It works a lot like Skype or Gtalk does on your computer, and we wouldn't be surprised if it gets built into the Android Gtalk client one day.
Until then, you need a third party VoIP (Voice over IP) account, and a little setup. Even without a clear and concise set of instructions from Google, we figured it out using the free service from sipgate after a bit of trial and error and tried to make it easy for you. Hit the forums for the How-To. There's a video of the new feature in action, find it after the break.
Having fun finding new and improved goodies in Gingerbread on your Nexus S or a custom ROM on your favorite device? Let us know any thing new, improved, or just plain cool that you find!