Every once in a while, it is a worthwhile exercise to step back and stop and think about the free software you use with Linux, day in and day out. The Linux kernel is only one part of it. There are literally hundreds of utility programs, the majority of which were produced by Free Software Foundation staff and volunteers. Linux is testimony to the idea that freely distributable software can be usable, and of high quality. You can help further the cause of the FSF in a number of ways, both directly and indirectly.
Serious volunteers are always welcome.
If you want to help support the FSF monetarily, you can do that too. 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 that expensive. Buying software and manuals directly contributes to the production of more, high quality, free software.