The best online listing of sleeper trains I know of is RailDude.
For example the Germany page http://www.raildude.com/en/germany/country/de lists all the night trains available passing through Germany, you can click on one to see the timetable, type of accomodation, prices etc.
seat61 has a lot of info but it's more of a guidebook than a listing of trains, and is focused on getting from the UK to places rather then getting around Europe.
Another option is to purchase a copy of the European Rail Timetable(search google for 'European Rail Timetable')
Finally, for checking specific routes go to http://reiseauskunft.bahn.de/bin/query2.exe/en and just search the route(the site is run by Deutsche Bahn but covers most of Europe), giving a departure time in the evening. Sleeper trains will come up as EN(Euronight), CNL(CityNightLine) or D in the Product Column