HyperPhysics theory:
- pendulum
- large swing pendulum
- air friction
- centripetal force
Do browse through the main HyperPhysics site for more general related topics; Newton's Laws, centripetal force, gravitational potential energy, simple harmonic motion, conservation laws...

University of Winnipeg physics notes and java applets:
simple pendulum physics notes

Bad Physics and Bad Science provide a witty counterblast to common scientific misconceptions.

Pendulums in clocks

Galileo and pendulums:
theories and clocks (http://es.rice.edu/ES/humsoc)
theories and clocks  (http://www.tau.ac.il/~museum - this page claims large amplitude pendulums have a shorter period - but it's actually longer).
invention of the pendulum clock (his son's account)
invention of the pendulum clock (with pictures of escapement mechanisms)

Experiments:
Galileo's Pendulum Experiments
experiments with single and double pendula
measuring the swing of a pendulum

Foucault's pendulum:
University of Guelph
California Academy of Sciences
University of Sydney

Amusement park physics - what you feel riding a pendulum

Java simuluations:
double pendulum
chaotic behaviour of sinusoidally driven pendulum
spring pendulum
chaotic pendulum in phase space
damped non-linear pendulum in phase space


Related materialworlds simulations:
Newton's balls
Pendulum clock
swing