Gimp Recipe: Background Noise
Open GIMP and create a new 100×100 image. Fill her up with a color, say a shade of grey.
Now, add some noise by selecting RGB Noise under Filters > Noise. Good values I have tried are 0.6 for all channels, anything higher than that would make the edges obvious when tiling for a page background. Also don’t forget Correlated noise for smoother edges during tiling.
Finally save your image and have fun. See here for a demo.