Bikes Not Bombs is a great organization. It collects unwanted bicycles, reburbishes them, and gets them to people who need them for transportation -- In Boston, Latin America, and Africa. It also teaches local kids how to bike and care for their bikes, and employs them in its stores.
I first learned about BnB when my daughter did the program, built her own bike and learned to get around Boston on her own. She also collected 40 bikes to donate to BnB - 40 bikes to pass along to others. 14 years later, she still uses her bike. (The same Trek she refurbished then!)
Basically, your donations enable BnB to fix bikes that aren't needed anymore by their current owners and send them to others who can make great use of them.