The Nexus One was ran out of the stock when it was first made available to the devolopers. Some still regard it as one of the best (and fastest) Android phones available. It's getting a bit harder to get as it seems that HTC isn't making enough of these bad boys to keep up with demand.
If you hurry though the Google Developer Store has some units available. It's been sold out until recently but thanks to a tip from Juan R. we were alerted this morning that they are once again available.
So go ahead, hurry up and get your Nexus on.