xkcd has some great t-shirts in the xkcd store. One of them, which I got in late 2014, is the uber-geeky "greek" shirt, though I don't see it in the official store right now.
For background, see Greek letters used in mathematics, science, and engineering (Wikipedia), though that doesn't have all the uses listed by Randall.
Here are the letters and uses presented on the shirt, row-by-row:
δ delta: dirac & kronecker deltas; isotope ratio
ε epsilon: a small number; strain; permittivity
ψ psi: quantum wavefunction, the planet Neptune
η eta: impedance, viscosity, efficiency
β beta: false negative rate, electrons, velocity in units of c
Γ Gamma: Gamma function, circulation, reflection
μ mu: coefficient of friction, micro-, permeability
λ lambda: half-life, lambda calculus, wavelength
σ sigma: standard deviation, stefan-boltzman
Ω Omega: death, universe density, ohm, sample space, the end
θ theta: angle
γ gamma: lorentz factor, photons
φ phi: 1.6180339887498948482045868343656381177
Σ Sigma: summation, terminal symbols, self-energy
ζ zeta: damping, the Rieman Zeta function
ξ xi: rieman xi, random variable, universal set, extent
ρ rho: resistivity, density, correlation
α alpha: right ascension, false positive rate, 1/137-ish
Φ Phi: flux, normal distribution CDF, 0.61803398874
π pi: 3.141592653589793238462643383279502884
ω omega: angular velocity, phonological word
τ tau: time, 2π, lifespan, torque
χ chi: mole fraction, electronegativity, chi distribution