When you say "design" are you talking about just the _design_ part, or the whole process (i.e. CAD, print, etch, and even soldermask and silkscreen)?
If just the CAD [layout] part, then refer to Mr. Bezaire's comment or consider Eagle CAD (only because of SparkFun's excellent tutorial [http://www.sparkfun.com/tutorials/108]) You can get it here:
http://www.cadsoftusa.com/downloads/freeware/?language=en..and here's a little more Eagle help:
http://hackaday.com/2009/01/15/how-to-prepare-your-eagle-designs-for-manufacture/If you also want to do the dirty work of actually fabricating the board:
* If it's because of the expense of getting it done commercially, then consider Batch PCB:
http://batchpcb.com/index.php/AboutUs* If it's because of the down and dirty aspect of it (or you're just a "hands on" kind of person) then there is a wealth of information behind the following Google searches:
- "make your own pcb"
- "toner transfer pcb"
- "photo resist pcb"
- "dremel pcb"
