Designer greats have been making cool mod furniture out of cardboard for decades. Usually, it's a little pricey for cardboard - you're buying the design philosophy more than the item. But there are some cool things being made out of cardboard these days that are fun and affordable:
Studio David Grass produces some affordable and attractive furniture like this sweet stoolCardboard Design makes all manner of products, but my favorite are their kid's playhouses. My own little rocketeer has enjoyed his for years :)
You can also find lots of affordable kids items, like these adorable chairs & table on Orange Onions.
And then again, you can quite easily make your own stuff! Sites like TreeHugger and Instructables give you great links and step by step directions to put your pile of cardboard to good use - give it a second life as toys and furniture good for guilt-free wear and tear.
[Pictured above, Foldaway Bookshop featured on Fubiz]