Denthor of Asphyxia's tutorials taught me quite a bit years ago http://archive.gamedev.net/archive/reference/listed82.html?c...

Dave Brackeen's VGA Programming in C tutorials also show off graphics modes and optimizations for things like primitive drawing, bitmaps, etc. I used this to create a game years ago http://www.brackeen.com/vga/index.html

There also used to be a site called ProgrammersHaven (later renamed ProgrammersHeaven that seems to have dumped all of its original content). This site had a collection of random programs and source files, many of which showed off demoscene techniques (parallax, plasma, 3D, bumpmapping, etc)