The best and only proper way to calibrate your screen is to use a hardward based screen calibration setup - a Spyder 3 is a good monitor calibration tool example. The reason you need to use a hardward based on like the syder, rather than the online "free" options is because the human eye is adaptive and subjective - ie our eyes will adapt to brightness and colour shifts which means that you cannot accuratly calibrate any screen with them.
Hardware there is not subjective and thus is an accurate and unbais way of calibration.
I should note though that laptop screens are not very good for calibration since they often have less control aspects and (you can easily see this for yourself) most will have a significant change in brightness and contrast if you change your viewing angle even a tiny bit. The best types of screen were CRT (the old big kind) but these days they are mostly off the market and the flatscreens (which all run way too bright out of the box for photography) also suffer form similarproblems as the laptop screens. A really good flatscreen though will not have the brightness and contrast shift based on your viewing angle - they do however cost a lot more.
A screen calibration though, even if limited, is advisable as it does help to give a more accurate image (though your eyes will have to adjust to it once done - things will seem really odd looking at first especailly when you compare calibrated to non-calibrated)
