All download links are below this document.

The Miditzer is a free virtual theater organ that is easy to install on the computer.
Jim Henry, the developer of the Miditzer, offers some versions for free.

The organ sounds beautiful and there are several soundfonts (organ sound sets) available for it.
The soundfonts are not VST samplesets but of the SF2 type. Fluisynth is often used for this.
Definitely recommended to start with.

There are 4 versions, 3 of which are free

- The Miditzer™ Style 150
- The Miditzer™ Style 160C
- The Miditzer™ Style 216

and the paid but beautiful Miditzer™ Style 260SP
You can order it for 100 US Dollar.
Compared to Hauptwerk a lot cheaper.

The Miditzer 216

Miditzer Style 260SP

Downloadable at: https://miditzer.org/

Hauptwerk is a professional organ simulator. 
Hauptwerk 4 has a free version, but unfortunately it can no longer be downloaded from the Hauptwerk website.

Hauptwerk is now releasing version 7. But if you can still take over version 4 from a friend or acquaintance, you can still get started for free.
Otherwise, consider jOrgan or GrandOrgue, for example.

However, the following sample sets are free to use. Barton and Morton do ask for a possible small donation, but that is not mandatory.

Paramount is a commercial sample set. Version 310 is free to download but of course has limitations.
If you want more, it quickly becomes pricey.

Download Organ sample sets (such as a theater organ. Paramount 310 is freeware, Barton and Morton donationware)

How to proceed. Look at this site. 

Paramount 310 theaterorgan

Barton 3/12 theaterorgan

Morton 3/10 theaterorgan

Above you see 3 virtual theater organs. These are free to download. 

A good example is a demonstration of Disney "When you wish upon a star".

This is played on the Paramount 332 theater organ.

This virtual sampleset is a payed version.

 Jorgan (Java virtual organ)

Jorgan is a JAVA programmed virtual organ. It's free and open source.
This means that no rights rest on it and anyone can improve the program.

The favorite Morton theater organ can also be installed in jOrgan. If you don't have Hauptwerk.

Downloadable at: https://sourceforge.net/projects/jorgan/files/

GrandOrgue

Grandorgue is also a good free alternative to the paid Hauptwerk.
The Barton theater organ, which also runs at Hauptwerk, is also available for the GrandOrgue program

GrandOrgue is downloadable at: https://sourceforge.net/projects/ourorgan/
The Barton sampleset for GrandOrgue is downloadable at: https://sites.google.com/site/virtualpipeorgans/home/barton-series-for-grandorgue

Genpo

Genpo or General Purpose Organ is a programming shell where you can transform XML files (special programming files that the enthusiastic programmer can create himself) into a Theater Organ.
Genpo is also free to use.

Genpo is downloadable at: http://genpo.sourceforge.net/

Finally, congratulations to the forces behind the "South African Virtual Organ Project" (Savirtual organs)

They have made beautiful theater organ samplesets which are supported by various free "Virtual organs"

But of course also the makers of Miditzer, jOrgan, GrandOrgue and Genpo. Thank you very much for offering such beautiful programs for free.

Below the complete download list: