University of St. Andrews (UK)
University of St. Andrews: Search for Physics Beyond the Standard Model
This webpage is a picture site showing how different leptons interact.
University of Colorado
University of Colorado: Physics 2000: Positrons, Alpha Particles, Gamma Rays
An interactive tutorial on positrons. Very readible and has a little extra for the advanced student.
Lawrence Berkeley National Laboratory
Berkeley Lab: The Particle Adventure: Leptons
This particle physics tutorial discusses the different types of leptons: the electron, tau lepton, muon lepton, and neutrinos.
Lawrence Berkeley National Laboratory
Berkeley Lab: Matter and Antimatter
This page examines the existence of matter and antimatter in nature.