Yes, there are.
This one is pretty basic. I see the kids slanted to the side often, so I think most of these are crap:
http://www.australiancyclist.com.au/article.aspx?aeid=12215
This one seems very useful:
http://www.amazon.co.uk/Bicycle-Follow-Me-Parent-Child-Trailer-Bike/dp/B001PH8APO
It folds up when you are biking alone
this page has more info in general
http://bikingwithkids.com/