Multi-day cruise experiences may vary. Try to find someone who has actually cruised on whichever ships you are considering.
Note that many of these firms are apparently agencies, as they are offering cruises on the same ships.
For smaller vessels, please visit the Hotel Boats and Barges page.
Extensive list of European River Cruises, from Cruise Critic. NOTE: this link will NOT take you directly to the list. First, follow the link above, then, on the resulting page, select "Europe All" in the first selection in the "Destination" box, then- "River Cruise" in the second selection in the "Lifestyle" box. Then click the "Search Now" button in the "Lifestyle" box. Whew!
Waterways World River Cruising - for river and canal enthusiasts; offered on behalf of Waterways World magazine
Canal & Company - 5 barges - France, Holland, Luxembourg, Germany
Croisieurope - numerous rivers (site in english and francais)
Peter Deilmann EuropAmerica Cruises - Danube, Elbe, Rhine, Rhone, Saone, Main, Mosell, Po
Noble Caledonia Ltd. - variety of ships and destinations
KD River Cruises of Europe -- Note: now owned by Viking River Cruises.
Sea Cloud Cruises - ship River Cloud - Danube, Rhine, Main and Moselle Rivers - 94 passengers
Viking River Cruises - operates 25 vessels; (web site in english, deutsch, francais, italien)
Voyages Jules Verne - France, Spain, Portugal, Italy, Sweden, Germany, Russia, Ukraine, Siberia(!)
Vaart! Reizen - Donau - (deutsch)
DDSG Blue Danube- (english, deutsch)
Maupin Waterways - Holland, UK, France, Eastern Europe, Italy
Paradise Cruise Centre Ltd. - "The Rivers of Europe"; Canada-based agency
Uniworld - USA-based river cruise agency
WorldWaterways - "the small ship experts"; USA-based agency
Elegant Cruises & Tours - USA-based agency
Naupar - sail and motor; also boat-bike (web site in english, deutsch, francais)