Agility is undeniably a sport designed with medium-sized dogs in mind... however, the Giants can have a great time at it too! You do need to take extra care when training a giant breed dog in agility, ESPECIALLY if they are younger than 18 months.
If you are interested in pursuing agility with a giant breed dog, there are some excellent articles to get you started at ChromaDane.
More photos on the Agility 2 page.
More information about doing Agility with Great Danes can be found on my blog.