Seems like they’ve released two or three different boxes per year, but not all are (were) available in all markets.
2011: The Live SMP and the Elements Play Gen 2.
2010: The Live Plus, Live Hub, and Elements Play.
2009: The WDTV Mini, the WDTV Live (Old version) and the WDTV HD Gen 2.
2008: The WDTV HD Gen 1