If you're looking for some free tools to help keep your monitor calibrated correctly, have a look at this list of 6 sites that offer advice and tools for free that'll help you with your calibration.
This site offers a basic aid for display / monitor calibration, care and maintenance. Instructions are provided on the first page then you simply work your way through the various tools supplied.
This site features a set of grayscale tones as well as shapes that feature tones at either end of the grayscale range. As the site says: 'Try it and see'. Just follow the instructions and have a look at the results after all, it's free!
3. Vanity.DK & FlatPanels.DK
This site features a test screen that shows various shades of grey and will help you see how well your monitor can differentiate close shades.
The Lagom LCD monitor test pages feature several images that you can use to easily adjust your monitor's settings to improve its picture quality. There also tests provided to help judge the overall image quality of a monitor.
This site features a simple test to check to see if your monitor's black point is set correctly and to also find the minimum shadow level it can display.
This site features serval quick checks that you can perform with the images provided. This includes colour, greyscale and gamma adjustments.
Visit the EIZO UK website.