Charities that accept bike donations
There is a Wiki with an International List of Bike Collectives and Community Orgainizations at BikeCollectives.org.

There are many other organizations, like the Salvation Army in the US, that take almost any item and either operate second-hand stores or sell the items to other thrift shops.  I prefer to deal with organizations like the Salt Lake City Bike Collective because I know the bike is going to checked well and repaired if needed before being returned to the community.