What are the files in the Blender/plugins directory? There are at least some C source files: clouds2.c and tiles.c in “textures”, blur.c, scatter.c and showzbuf.c in “sequence”… are they usable if compiled? There are also makefiles and .so:s for the .c:s
The .c files compile to the .so files - that is, they’re already compiled 
These are texture and sequence plugins. While I can’t comment on the sequence plugins (never touched 'em), the texture plugins allow extra procedural textures to be used.
Try this:Start a new blend, delete the plane, add a sphere.
Add a new material to the sphere, then add a new texture to the material.
Next, go to the Textures window(F6), press Plugin, then the Load Plugin button that appears.
This opens up a file selection thingy - find the texture plugin directory, and middle click on one of the .so files.
You should now have a bunch of new buttons and options. Play with them
Add a lamp, render
You can find some more info here, as well as lots of really cool plugins:
http://www-users.cs.umn.edu/~mein/blender/plugins/
http://www-users.cs.umn.edu/~mein/blender/plugins/texture.html