You may be hard pressed to find places that accept donations of used pillows for sanitary reasons. However, local animal rescue or wildlife shelters might be interested (or you could turn them into pet beds and donate those). Some animal shelters will not take pillows as it would require too much effort to repeatedly wash them, and each place has it's own set of rules and needs, so please check to find out.
You could also give them away via a classified ad or material exchange site to someone who could get a little more use from them.
Foam pillows might be best taken to mattress or foam recyclers.
Look here for more ideas http://earth911.com/home-garden/7-ways-to-reuse-and-recycle-old-pillows/
If your pillows are feather or down, they can be emptied into a compost bin and the outer cover recycled with textiles.
No need to toss old ones in the garbage, as some charity stores will accept well worn items to recycle for the fibers. They bale them up and send to a cloth recycler, to be processed into other useful items. The charities are paid per pound for these textiles (It's a good way for them to make money and keep these type of things out of landfill). There are very few circumstances where textiles should ever be discarded to landfill (textiles contaminated with oil and chemicals being the exception).
When dropping them off, let the staff know they are intended for textile recycling.
