City Sightseeing
Hop-on, hop-off tours of Paris by open-top double-decker bus.
http://www.city-sightseeing.com/
http://www.guidefriday.com/

Cityrama
An open-top bus tour of Paris is just one of many things this company has to offer.
http://www.cityrama.fr/ http://www.graylineparis.com/

Paris L'Open Tour
Offers a selection of open-top bus tours of Paris.
http://www.paris-opentour.com/index_eng.htm

PIDF.com
Index of firms running guided bus tours of Paris.
http://www.pidf.com/an-world/u_1/home13.asp