Apogee released the (Caves of) Thor Trilogy as freeware (without support) December 23rd, 2005, along with a number of other moldy oldies from their back catalogue. The registered set can be downloaded at ftp://ftp.3drealms.com/freeware/thor.zip
Like most Apogee games the Thor Trilogy is a trilogy. The first part of the trilogy is shareware, the two other parts are not. The names of the three episodes are:
1. Caves of Thor 2. Realm of Thor 3. Thor's Revenge
The Thor Trilogy was the first game Apogee released that was from another author.
You could buy the source code of this game for $195.