Get Pixel out of the Screen(2d)

Hi!
How do I get the pixelvalue out of the screenbuffer using OpenGL?? e.g.
int pixel = GetPixel(x,y); ??
Thnx,
Blob

glReadPixel(); http://www.eecs.tulane.edu/www/graphics/doc/OpenGL-Man-Pages/glReadPixels.html

Chris