Sources of digitized passenger lists from steamship lines:
1. Gjenvick-Gjønvik Archives (http://www.gjenvick.com/)
Great archives, great site! They have a number of passenger lists already transcribed, and are currently working on adding more transcriptions. I love seeing the artwork on the covers of the steamship lines.
Passenger Lists - Holland America Line - Available at the Gjenvick-Gjønvik Archives
They have approximately 50 lists from the Holland America Line, but they don't have the Volendam leaving Rotterdam on 8 August 1950.
2. FamilySearch.org
Register for free access to this comprehensive site, and let the searching begin! I've used this to search census, birth, marriage, and death records. They are in the process of adding digital images of their extensive microfilm holdings. I discover something new with each exploration. Just a few days ago, I found that they have (or will have) images from steamship lines' passenger lists, including:
Netherlands, Passenger lists Holland-America Line, 1900-1974
The Family Search description of this particular collection: "Passenger lists are available for the following range of dates: 3 May 1900 through 14 October 1974. Original records are available through the Rotterdam City Archive (Gemeentearchief te Rotterdam), Netherlands. This collection is being published as images become available."
Currently, to explore the records, the file directory is in Dutch: (Learn a little Dutch with me...)
Netherlands, Passenger lists Holland-America Line, 1900-1974
Van continent-Naar continent > Europa-Noord Amerika
Vertrekjaar > 1950
Vertrekmaand en-dag > aug. 8
Naam Schip > SS Volendam (No images yet...)
There are 113,050 images from this collection, but they do not yet appear to be available, even with a Family Search login. Either the images aren't ready to be released yet, or tt might be possible that they can be accessed within a Family History Center.
That's all I've found for now. I'm still exploring different ways of building a virtual digital collection for this blog.