Most of the demos were downloaded from the original (now defunct) SDL website, and some random places on the internet. I don't know any of the authors and have no idea what the licences are, unless they are included in the archives.
I have manually cleaned up the projects to the point that they compile on my rig.
Most links to original sources are available on web archive only.
All projects can be compiled with gcc and make both on Windows and Linux.
Original link: https://gafferongames.com/post/fix_your_timestep/
Mirror (code): ggl
Original link: http://www.koonsolo.com/news/dewitters-gameloop/
Mirror (code): dwl
Original link: http://www.brandonfoltz.com/downloads/tutorials/The_Game_Loop_and_Frame_Rate_Management.pdf
Mirror (code): gml
Original link: http://www.ouhlavache.com/stv/tunnel2d.tar.bz2
Mirror (code): 2d_tunnel
Original link: http://www.libsdl.org/projects/bump
Mirror (code): bump
Original link: http://www.libsdl.org/projects/fire
Mirror (code): fire-1.0
Original link: https://www.libsdl.org/projects/pipenightdreams/
Mirror (code): plasma-1.0
Original link: https://www.libsdl.org/projects/PTC/
Mirror (code): PTC-demos-1.0
Original link: https://www.libsdl.org/projects/sierp/
Mirror (code): sierp-1.0
Original link: https://www.libsdl.org/projects/warp/
Mirror (code): warp-1.1
Original link: https://www.libsdl.org/projects/water/
Mirror (code): water-1.0
Original link: https://www.libsdl.org/projects/xflame/
Mirror (code): xflame-1.0
Original link: http://home.uninet.ee/~ragnar/waves/
Mirror (code): waves-1.2
Original link: http://www.angelfire.com/ri/nctv8/lightning.tar.gz
Mirror (code): lightning
Original link: https://www.libsdl.org/projects/stars/
Mirror (original code archive): stars-1.0.tar.gz
Mirror (code): stars-1.0
Original link: http://www.syntheticsw.com/~wizard/projects/birthday/
Mirror (code): birthday
Original link: http://www.2dgame-tutorial.com/sdl/fading.htm
Mirror (code): fadingv3
Original link: http://www.libsdl.org/projects/flxplay/
Mirror (original code archive): flxplay-0.2.tar.gz
Mirror (code): flxplay-0.2
Original link: http://gpwiki.org/index.php/Files:Sdl_collide.zip
Mirror (code): sdl_collide
Original link: http://members2.jcom.home.ne.jp/shirowmiura/BlitPool/index_en.html
Mirror (code): SDL_BlitPool
Original link: https://kennycason.com/posts/2009-01-02-sprite-class-in-c-using-sdl.html
Mirror (code): sdl_sprite
Original link: http://gpwiki.org/index.php/Files:SDL_bouncing.zip
Mirror (code): bouncing
Original link: http://www.libsdl.org/projects/cmouse
Mirror (code): Mouse
Original link: https://www.libsdl.org/projects/newvox/
Mirror (code): newvox-1.0
Original link: https://www.libsdl.org/projects/scrap/
Mirror (code): scrap-1.0
Original link: https://www.libsdl.org/projects/screenart/
Mirror (code): screenart-1.1
Original link: http://burningsmell.org/SDL_plattest/
Mirror (code): SDL_plattest
Original link: http://gpwiki.org/
Mirror (code): C_CustomResource
Original link: http://gpwiki.org/index.php/Files:SDL_ResourceLoader.zip
Mirror (code): SDL_ResourceLoader
Original link: http://gpwiki.org/index.php/Files:Sdl_ogl.zip
Mirror (code): sdl_ogl
Original link: http://burningsmell.org/sdl_wingman/
Mirror (code): sdl_wingman-0.2
Original link: http://www.easyfish.de/sdl_with_gtk.tar.bz2
Mirror (code): sdl_with_gtk
Original link: http://www.glop.org/software/sotb/
Mirror (code): sotb-0.2.0
Original link: http://sourceforge.net/projects/spiralizer
Mirror (code): spiralizer-2004-04-20
Original link: http://www.cloudsprinter.com/random/nodchod.zip
Mirror (code): nodchod
Original link: http://sdl.kill.mine.nu/phire/
Mirror (code): phire
Original link: http://rhk.dataslab.com/roaddemo/
Mirror (code): roaddemo-1.0.1
Original link: http://mars.sourceforge.net/down.php?dow=e
Mirror (code): SDL_mouse_tile
Original link: http://www.marsnomercy.org/cms2/content/view/21/78/
Mirror (code): SDL_sinus_font
Original link: http://www.marsnomercy.org/cms2/content/view/21/78/
Mirror (code): SDL_mix
Original link: http://www.marsnomercy.org/cms2/content/view/21/78/
Mirror (code): SDL_iso_mouse
Original link: http://www.marsnomercy.org/cms2/content/view/21/78/
Mirror (code): SDL_mouse_scrolling
Original link: http://www.marsnomercy.org/cms2/content/view/21/78/
Mirror (code): GL_blit
Original link: http://www.marsnomercy.org/cms2/content/view/21/78/
Mirror (code): SDL_astar_pathfind
Original link: http://encelo.netsons.org/
Mirror (code): PNG_savesurf
Original link: https://www.autistici.org/encelo/prog_sdldemos.php
Mirror (code): SDL_demo
Original link: http://olofson.net/download/
Mirror (code): parallax-2
Mirror (code): parallax-3
Mirror (code): parallax-4
Original link: http://olofson.net/download/
Mirror (code): smoothscroll-1.1
Original link: http://olofson.net/download/
Mirror (code): ballfield-1.0
Original link: http://olofson.net/download/
Mirror (code): pig-1.0
Original link: https://su2.info/demos/
Mirror (code): blur-0.1
Original link: https://su2.info/demos/
Mirror (code): flares-0.4
Original link: https://su2.info/demos/
Mirror (code): fire-0.3
Original link: https://cgit.freedesktop.org/~joonas/cairosdl/
Mirror (code): cairosdl
Original link: https://www.libsdl.org/projects/optimum/
Mirror (code): optimum-1.0
Original link: https://code.google.com/archive/p/sdl-gweled/downloads
Mirror (code): OilWar-1.2.1
Original link: http://furball.schemm.lssu.edu/Twigs_In_Space/
Mirror (code): twigsinspace
Original link: http://furball.schemm.lssu.edu/SDL/
Mirror (code): TwigMan
Original link: http://www.libsdl.org/projects/aliens
Mirror (code): aliens-1.0.2
Original link: http://gpwiki.org/index.php/Files:EatCppSDL.zip
Mirror (code): EatCppSDL
Original link: http://gpwiki.org/index.php/Files:KySoko.zip
Mirror (code): KySoko
Original link: http://gpwiki.org/index.php/Files:Paddles-lin.tar.gz
Mirror (code): paddles
Original link: https://sourceforge.net/projects/sdlinvaders/
Mirror (code): SDLInvaders
Original link: https://www.scs2.net/next/index.php?id=200
Mirror (code): SDLMemory
Original link: http://web.ukonline.co.uk/james.blewitt/jetpac.html
Mirror (code): jetpac-0.2.5
Original link: http://jeuxfr.free.fr/Ufo/
Mirror (code): GNUfo
Original link: https://greg-kennedy.com/wordpress/2019/05/01/modest-bricks/
Mirror (code): modest
Original link: http://code.google.com/p/maze-rays/
Mirror (code): maze-rays-1.1
Original link: http://www.arcasas.es/lagunex/descargar.php
Mirror (code): lagunex-1.0.orig
Original link: http://magigames.org/moonlander.html
Mirror (code): moon-lander
Original link: https://sourceforge.net/projects/sdlscavenger/
Mirror (code): sdlscav-145