You could use them to create a current in your tank...to help with the dead spots, and have the power filter... or any kind of filter to collect debri.
You could use them to power undergravel filters, although I find no use for them if you have another kind of filter.
i bought a second hand tank the other day and the guy gave me a power head with a piece of tube he used for an automatic syphon.
he literally put the powerhead in the tank, attached the hose to the output and pluged it in. sucks the water out in no time..
i wont use it though as i like to gravel vac instead.
the question is could i use this powerhead for anything else? a diy filter maybe? it's never going to be used as a syphon now and seems a waste leaving it in the cupboard.