Here are a few essential links for anyone looking to design their own effects circuits. These are just a few of the hundreds of pages that are useful and are the ones I tend to use most often.
http://www.electronics2000.co.uk/calc/reactance-calculator.php – Capacitor Reactance Calculator
http://www.muzique.com/lab/main.htm – Jack Orman’s brilliant Lab notes
http://www.muzique.com/schem/filter.htm – RC Filter Cut-off calculator
http://www.muzique.com/lab/buffers.htm – Basic Buffer circuits
http://www.geofex.com/ – R.G Keen’s excellent website
http://www.jiggawoo.eclipse.co.uk/guitarhq/Circuitsnippets/snippets.html – Tim Escobedo’s Circuit Snippets
http://www.pisotones.com – Some nice projects and build reports.
http://analogguru.an.ohost.de/techstuff/Reactance.htm – Capacitor Reactance Table
http://www.justradios.com/uFnFpF.html – Capacitor conversion table
And obviously – the forums – http://www.freestompboxes.com and http://www.diystompboxes.com