There's also www.dumpandrun.org.  I haven't used them but from the website, it looks like  a good way to organize the end of year clear out and turn it into cash for a good cause.

I live on campus at a boarding school and am part of our annual clear out.  Here's what we do:

We collect clothes/shoes for a local charity and to be sent to Haiti.

We use MA-based www.gotbooks.com for collection of our books on campus and they pay by the ton for the amount you collect.  They are a for profit but they do support local non profits, teachers, schools etc.  They send some of their books to troops abroad and to replenish libraries in need, such as those affected by Katrina.

We collect unwearable sneakers and take them to Niketown store in Boston  for Nike-Reuse-a-Shoe to turn into sports surface and

we collect batteries and bulbs to be safely disposed via our facilities dept.