Besides showing off the new hubs, multitasking, IE9 and Skydrive support coming later in 2011 (and those were only the announced features, there's evidently a lot more), there was one surprise app that we saw: Groupon (see their site)
For those who haven't been following, Groupon is the biggest "deal" website on the 'net, helping consumers find the best offers and lowest prices for a variety of items and services. They're also known for turning down a $6 billion offer from Google to buy them out. Yes, we said billion.
Anyways, this super service is coming to Windows Phone in an official capacity sometime soon--and it even features a Live Tile with updated deals (similar to that just-as-awesome Slick Deals app available now). All we know is this is great news for WP7 to land yet another major brand.