New in .NET 3.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace SeqMap
{
    class Program
    {
        static void Main(string[] args)
        {
            int[] a = { 1, 2, 3 };
            int[] b = { 3, 4, 5 };

            var r1 = a.Map(x => x * x );
            var r2 = a.Map(b, (x, y) => x + y);

            foreach (var item in r2)
                Console.WriteLine(item);
        }
    }

    public static class Seq
    {
        public static IEnumerable<T> Map<T>(this IEnumerable<T> a, Func<T, T> fun)
        {
            foreach (var x in a)
                yield return fun(x);
        }
        public static IEnumerable<T> Map<T>(this IEnumerable<T> a, IEnumerable<T> b, Func<T, T, T> fun)
        {
            foreach (var x in a)
                foreach (var y in b)
                   yield return fun(x, y);
        }
    }
}