In addition to the Nexus One you can purchase an Android Dev Phone from Google.
http://developer.android.com/guide/developing/device.html#dev-phone-1
If you want a SIM-unlocked phone, then you might consider either an Android Dev Phone or the Google Nexus One. Both are SIM-unlocked so that you can use them on any GSM network using a SIM card. The Android Dev Phones also feature an unlocked bootloader so you can install custom system images (great for developing and installing custom versions of the Android platform). To purchase a Nexus One, visit google.com/phone. To purchase an Android Dev Phone, see the Android Market site (requires a developer account).
A developer account costs $25, but last I checked the dev phones are $400. So it should end up being cheaper than a Nexus One.
There are two dev phones, one is basically the original G1, the other is an HTC Hero I think...