Here's a pretty simple prime number generator I used for a problem in ProjectEuler (specifically, calculate the sum of every prime below 2,000,000, although this can be generalized to sum every prime below any integer)

sumofprimes.rb

def sumofprimesbelow number
	prime_array = [3,5]
	current_num = 7
	while current_num < number
		prime_array.each do |factor|
			if current_num % factor == 0
				break
			elsif factor > Math.sqrt(current_num)
				prime_array << current_num
				break
			end
		end
		current_num % 10 == 3 ? current_num += 4 : current_num +=2
	end
	
	prime_array.reduce(:+) + 2
end

Revision 2 is much better as it cuts an entire array out of the process. Not sure if it's still as efficient as it could be, but it works MUCH faster than Revision 1 for large numbers.