Better way of calculating Project Euler #2 (Fibonacci sequence)
No iteration is required to calculate this result.

Every third Fibonacci number is even.  The Fibonacci numbers can be expressed in closed form as: 

Fib(n) = 1/sqrt(5) * (phi^n - psi'^n)


where

phi = (1 + sqrt(5) / 2)


and 

psi = (1 - sqrt(5) / 2)


F(n) is even when n is a multiple of 3.

Therefore the sum of even fibonacci numbers is equal to the sum of two geometric series, and can be calculated directly and exactly.

P.S. A little experimentation shows that

sum(i=0..n) Fib(3*i) = (Fib(3*n + 2) - 1) / 2


e.g. 

2 + 8 + 34 = 44 = (89 - 1) / 2