The Nintendo Channel
One of my favorite features on the Wii is the Nintendo Channel.
Besides watching videos and exploring the game library, there is a DS Download Service in which you can quickly download demos of DS games right to your DS.
Many times you can even purchase the DS game right through the Wii, which is really impressive (it takes me to the Gamestop website game detail page)
I wish that gamers would be able to download WiiWare and Virtual Console demos too..