The Golden Ratio is (sqrt(5)+1)/2, often denoted φ (phi), or about 1.618033988749894848204586834365638117720...
Mathworld -- Golden Ratio
Fibonacci Numbers -- whose successive ratios approach the Golden Ratio
Recurrence Cycle -- A "Golden" Recurrence Relation -- un+2 = 1/(un un+1φ) has a period of 5.
Trig functions of Special Angles -- The golden ratio pops up here, too.
The webmaster and author of this Math Help site is Graeme McRae.