There are several excellent organizations that you could donate your old computers to:
Freecycle is an online free recycling/gifting movement designed to keep tangible items out of landfills....a lot of computers get tossed out and there are many people who can use an old computer or a part from that computer.
Computers for Kids will pick up your old computer, do any refurb needed, and set it up as a student computer at some elementary school. They'll give you a tax deduction as well.
Geeks Without Borders works to provide computer resources to
impoverished communities:
Gifts In Kind distributes donations to recipient organizations serving a wide spectrum of people in need, including: at-risk youth, the homeless, older Americans who are struggling or ill, low-paid military families, victims of domestic abuse, refugees, immigrants, the illiterate, the under- and unemployed, victims of natural and man-made disaster, and many others.
Gazelle will take you old Macs even if they have no value. They will recycle and triple wipe your data.
In some big cities the Goodwill has centralized stores that carry nothing but old computer equipment. As a last resort it might be worth seeing if your city has such a Goodwill store.
e.g.:
Apple has a Reuse and Recycling Program.
The service allows anyone to send their old computers and monitors to Apple to be recycled for free -- regardless of brand.
You may even get an Apple Gift Card for your old equipment.