If you have unlocked your Nexus One (using "fastboot oem unlock") then you can build Froyo from AOSP and install it yourself (or you can download and install a

If you have unlocked your Nexus One (using "fastboot oem unlock") then you can build Froyo from AOSP and install it yourself (or you can download and install a build by someone else if you trust them). If you do not want to unlock your phone, then you will need to wait for a signed OTA update from Google. The AOSP build does not include any of the Google apps or frameworks (GMail, Google Maps/Navigation, Market...). You either need an official Google build for that, or you need to use one of the methods to copy those apps from a image that has them.