L2TPv3 is a great feature for extending a Layer2 network across sites.  This is very useful when you are using Live Migration with HyperV or when you want to pass traffic (at Layer 2) to a different location. 

See http://www.mikepoulson.com/archive/2010/09/10/l2tpv3-performance-tweaks-for-geocluster-live-migration.aspx for the full post.