Monitor calibration

On a properly calibrated monitor, the central square below should be the same shade of grey as the background;
at the same time, the eight dark and eight light steps on either side should be clearly distinguishable.

Before calibrating the monitor, start by selecting the factory default values for "Brightness", "Contrast" and "Colour temperature" via the On Screen Display (OSD) controls of your monitor.

How to calibrate your monitor:
By eye - Least precise method, but still better than nothing; requires basic calibration software, e.g. Adobe gamma
Software calibration - Much better and more consistent results; requires a measurement device ("spider"), e.g. ColorVision Spyder, Eye One Display, …
Hardware calibration - Best and most precise results; requires a high-end monitor with proprietary software (e.g. EIZO Color Navigator) and a measurement device ("spider")

Colour Temperature
5000 K - Classical "pre-press" setting (useful to approximate printed results, but often subjectively a little too yellow)
5500 K - Best subjective match to prints viewed under bright gallery lighting (also, perfect correspondance to Daylight balanced slide film)
6500 K - sRGB standard (useful to approximate average web viewing conditions, but often a little too blue to closely match printed results)

Gamma
1.8 - Old "pre-press" standard for MACs, now superseded by 2.2
2.2 - sRGB standard, now universally accepted for both PCs and MACs

Luminance (= white level)
80 cd/m2 - Classical "pre-press" setting, works best for work in dim rooms, and corresponds to prints viewed in relatively subdued lighting
100 cd/m2 - Acceptable alternative setting, works well for work in medium-lit rooms, and provides a good subjective match to prints viewed under bright gallery lighting
120 cd/m2 or above - Suitable for general computer work in bright rooms, but best avoided for critical photography work (too bright with respect to printed output)