My family uses backpacks for all types of reasons. My kids use them for school. My husband uses a few for hiking. He has one for hiking for a few hours and a bigger one for overnight trips. I used a small one when I was travelling in Slovakia last summer. I have a bigger one for my good camera and photography equipment.
That's a lot of backpacks. I found a few programs that are willing to take gently used backpacks.
The American Birding Association takes adult sized backpacks for their scientists to use in the field and carry equipment. The address is Birder's Exchange, American Birding Association, 4945 N 30th St, Colorado Springs, CO 80919 or you can visit their website for other items they need donated. www.aba.org. I was having trouble with their "items to donate" link.
The Aspire, Inspire Foundation in California has a program where they donate new and gently used backpacks to kids who can't afford them. Their website is http://www.aspireinspire.org/backpack.html.
The Orphan Outreach Mission gives gently used backpacks to children overseas. Their website is http://www.missionbackpack.org/faq/.
These are just a few of the programs available. You can choose which one works best for you. Your local thrift store, the Salvation Army, and Goodwill will also take them.