Calibrating your monitor. What can I say? We all try to get away with calibrating our monitor by eye, but that only works for so long and unfortunately our eyes change. Not just over the years but throughout the day. We get use to seeing a certain way and we adapt in different lighting situations. In the end our paintings, photos or what have you suffer. Our darks are too dark, the colors are to saturated or the hues just don't look right especially when we go to print. So we are left to buying hardware to calibrate our monitors.
So even if you are just a hobbyist, I suggest using hardware to calibrate your monitor (on a regaler basis) to get the most out of your creative efforts. Going by eye alone just doesn't cut it forever.
Hope this helps,