It is only good sportsmanship and fair play to “give something back” to the organization that has done so much for you: the FSF. You can help further the cause of the FSF in a number of ways, both directly and indirectly.
If you are a programmer or a writer, or both, the FSF has software and documentation that needs to be written. Serious volunteers are always welcome.
If you want to help support the FSF monetarily, you can do that too. You can buy software and/or documentation from them. The FSF sells tape and CD-ROMs with their software on it. You probably already have most of the software, but you may wish to have the printed documentation that goes with it. The GNU manuals are nicely printed and bound, and are not expensive. Buying software and manuals directly contributes to the production of more, high-quality, free software.
In the U.S., you can make tax-deductible donations to the FSF. It is considered a non-profit organization under U.S. law.
Indirectly, you can choose to buy your Linux distributions from resellers who state that they give a percentage to the FSF. If your favorite distributor does not do this, then ask them why they don't, and encourage them to do so.
Consider what you can do to help the FSF, and then do it!
The FSF can be reached at:
Free Software Foundation, Inc.
675 Massachusetts Avenue
Cambridge, MA 02139-3309 USA