Einstein's special relativity is formulated in terms of 4-D commutative hypercomplex mathematics. The traditional results are obtained, but some additional effects are suggested.

This derivation uses the group property of the Lorentz transformations, which means that a combination of two Lorentz transformations also belongs to the class Lorentz transformations. [PDF]

The fundamental equations of special relativity are derived with only high school algebra and toy universes consisting of moving rulers. [PDF]

Expressions for momentum and energy of a relativistic particle may be derived from the composition law for velocities along one spatial dimension.

The usual relativistic expressions for momentum and kinetic energy are generalized from the one-dimensional to the three-dimensional case.

The exact equation for the Doppler shift in a uniformly accelerating rocket is derived in two different ways. The first method depends on a functional equation and Einstein’s approximation. The second approach is a direct application of several familiar equations in the relativity of uniformly accelerated motion.

A multimedia tutorial on Special Relativity. The introductory level takes 10 minutes, but has links to over 40 explanatory pages giving greater depth and breadth.

An article from the Wikipedia encyclopedia.

The general problem of relativistic addition of velocities – and the successive application of noncollinear Lorentz boosts – is addressed. [PDF]

This is chapter 1 of a book by Chris Doran and Anthony Lasenby on geometric algebra, which is the natural mathematics of spacetime. [PDF]

Here is the formula for adding velocities in special relativity when motion occurs in a single direction.

The major principles of special relativity (SR) are discussed in an accessible way, via 5 segments, to help you understand the lingo and theories involved.

There is a preferred algebra of quaternions and complex numbers that is ideally suited to express the equations of special relativity and classical electrodynamics. [PDF]

A growing collection of pages on special relativity, including Special Relativity in under 15 Minutes!

The online physics course notes for Physics 3063, by Professor Rick Field, University of Florida, is a good summary of many of the useful formulas used in special relativity.