This is an extremely simple m-file which implements the method described in :
J. Immerkær, “Fast Noise Variance Estimation”, Computer Vision and Image Understanding, Vol. 64, No. 2, pp. 300-302, Sep. 1996
The function inputs a grayscale image I and returns Sigma, the noise estimate. Here is a sample use:
I = rgb2gray(imread('sample.jpg'));
The advantage of this method is that it includes a Laplacian operation which is almost insensitive to image structure but only depends on the noise in the image.