I was wondering if there is any other way to crystallize materials from a solution other than using supersaturation?
No. Per definition you need to have at least local supersaturation. You can archive it by different means (cooling, evaporating of solvent, mixing with other solvent...) but in the end you need supersaturation. See: http://xray.chem.ufl.edu/growing%20tips.htm or http://web.mit.edu/x-ray/cystallize.html
Of course you can use techniques like sublimation but your question was about solution (btw in case of sublimation you also have supersaturation, only difference is that your solvent is gas).
Not really. By definition, if some solute is in a solution below the saturation point at that temperature, it's going to remain in solution. The only way to get it out is to lower the temperature or evaporate some of the solvent to increase the concentration.
If you don't care especially about the integrity of the chemical, you could always precipitate it with some other reagent, but then of course your crystals would not be of the chemical you started with, but perhaps could be purified through some other means...