One clever individual managed to find the Android 2.2 Froyo .zip file on Google servers and posted it to the public. If you are running a Google Nexus One “non-rooted stock ERE27 system” then you can simply download the file off the Google server and run it on your Nexus One. In case you have a stock Nexus One that has never been rooted, you can still run Android 2.2 using a method explained here.
In case you decide to install the latest Android software, proceed with caution and only do so if you know what you are doing. Feel free to let us know how it works in the comments below.