You cannot do any real calibration without extra hardware. Bare minimum is a colorimeter but you really want spectrometer. Unfortunately, spectrometers come with high costs (second hand devices cost at least $150) and colorimeters not really suitable for LED backlighting which majority of the LCD displays are using nowadays. The ColorHug2 is claiming to be an exception because it uses much higher quality sensor than typical colorimeters.
The best you can do without any extra hardware is to adjust monitor settings according to test images (e.g. http://www.lagom.nl/lcd-test/) and then adjust gamma ramp by running xgamma -gamma <some number between 0.5 and 2.0> until the gamma test image reads that the actual gamma is 2.2. Figuring out the correct number for the -gamma argument is trial and error. Try 1.0 first and if gamma is below 2.2 according to test image, you have to decrease your current gamma value. (My monitors have always used xgamma gamma between 0.9 and 1.2 but low quality displays can be all over the place. My "factory calibrated" monitor requires 0.92.) After that, just pretend that your display is a calibrated sRGB monitor. If you have a laptop where only display adjustment available is the brightness of backlight, there's really not much to do.
And if you're happy with your display currently, do not visit http://www.lagom.nl/lcd-test/ because the more you understand the flaws in your display the less happy you will be...
