In the past I would use Ferrylines (see below), but this website has been down for a while now, promising to come back online soon. Until that happens, I use Openstreetmap:
- Browse to Openstreetmap Turbo to search for ferry routes
- Navigate the map to the area of interest
- Click "Run"
- When warned for "Large amounts of data", click "Run anyway"
The result might look something like this:
Openstreetmap rendering of ferry routes.
It does not discriminate between overnight ferries and short ferries, but typically, longer routes run overnight and shorter routes run during the day. You can check out details by clicking on a route. With some luck, there is enough information there such that you can use a web search engine to get additional details.
As per this comment by asdfex, the difference between blue and violet lines is not relevant (route represented as a single way or a collection of ways).
Like with other web sources, information may be out of date or incomplete. On the plus side, when information is out of date or incomplete, you could go and edit Openstreetmap yourself to remove or complement it.
It should be possible to run more advanced queries and select ferries based on particular characteristics (as long as those characteristics are included in the Openstreetmap database). This is technical and works via the Overpass API query language. Related questions can be asked at the GIS Stack Exchange.
Update March 2023: The ferrylines.com website is currently down, apparently for maintenance, but is supposed to come back "in short time".
Original answer from 2017
In my experience, the most up-to-date source for ferries in Europe and in fact around the world is ferrylines.com. They have a reasonable good interface.
Although you cannot specifically search for overnight ferries, a good hint for “overnight’ is that the crossing lasts at least 6–8 hours and that it operates at most 1–2 times per day. Also, (almost?) all overnight ferries are primarily ferries for cars and trucks, but do also permit passengers (some roll-on roll-of (RORO) ferries accept only trucks, but you'll find those mostly on freight ferry websites). Even the Cunard Line passenger liner from Southampton (or Hamburg) to New York is listed, as are, for some reason I don't quite understand, Eurostar trains.
Snapshot from Ferrylines.com. My apologies for the low resolution image; I could not get it under 2 MiB otherwise.