Any tool to tell me the exact color value at a point on a png image?

Question

I want to get the exact color value(RGB, HSB, or any other color space value that can be converted to RGB value) at any given point(preferably the point under the cursor) on a png image. Any tool for that? Thanks.

PS: To be more specific, I'm not asking for screen color picker, but PNG image color data picker. General screen color picker can not perfectly pick the exact color value in a PNG image, for example, alpha channel is missing.