ADDITIONAL FPS GAMES - OTS SOFTWARE
Here are some additional FPS games from a developer called "OTS Software." Dates of games & other information was taken from a Wikipedia entry (LINK: https://en.wikipedia.org/wiki/OTS_Software). All the games are freeware, according to the website. I could not find these games on this website. NOTE: I spotted that this website already had listings for 3 of the games (partitioned below) partway through my research. I'm pretty sure that this website doesn't have the first 4 games on the list but I'm not certain.
COMPANY WEBSITE:
http://www.otssoftware.com/
THE CRYPT (2003 FPS)
http://www.otssoftware.com/CryptFull.aspx - website
https://www.youtube.com/watch?v=NH7xvZUNivY - Game play video
INSIDE THE BEAST (2003 FPS)
http://www.otssoftware.com/Beast.aspx - website
https://www.youtube.com/watch?v=R50ycdTf0s8 - Game play video
DR. GREEN (2004 FPS)
http://www.otssoftware.com/DrGreen.aspx - website
https://www.youtube.com/watch?v=MwTnnOfE96g - Game play video
MEDUSA'S LAIR (2007 FPS)
http://www.otssoftware.com/Medusa.aspx - Website
https://www.youtube.com/watch?v=1YM9bfcRfC4 - Game play video
The following games are already listed on this website as "Free Fan-Made games." I'm not sure what designates a game as "Free Fan-Made." They are included as-is primarily because I did not spot them on this website initially when writing down websites for these games.
ALIEN EXTERMINATOR (2006 FPS according to Wikipedia website, 2008 on this website).
http://www.otssoftware.com/AlienExterminator.aspx - website (also an alternate download site and screenshot)
https://www.youtube.com/watch?v=gWgNae6mnyg - Game play video
ARENAXCAPE (2008 FPS)
http://www.otssoftware.com/ArenaXcape.aspx - website (also an alternate download site and screenshot)
https://www.youtube.com/watch?v=nhloH2e13x0 - Game play video
ALIENXCAPE (2008 FPS)
http://www.otssoftware.com/AlienXcape.aspx - website (also an alternate download site and screenshot)
https://www.youtube.com/watch?v=nhloH2e13x0 - Game play video