There are 2 approaches to monitor calibration. One is to use a software program - if you have Photoshop, Adobe Gamma is perhaps as good as you can get for software, but I think there are a number of freeware packages available as well.
These are not too bad for setting the black and white points of your monitor, and may address the problem you are having with dark images.
But software alone is not very good for getting colors spot on, as although they allow color adjustment, the process is very subjective. For color you need one of the hardware devices mentioned above which are capable of setting colour in absolute terms.
In terms of investment, monitor calibration is not a one off task - probably should be done monthly, so the hardware actually works out as good value for money over time.
Colin K. Work, Pixstel