Takes simple tasks and makes them so brutally clean. Simple examples…
int [] i = { 1, 3, 5, 6,7, 9 };
int [] i2 = { 2, 4, 5, 8, 10, 3, 5 };
// Combination of all (repeats included).
IEnumerable<int> concat = i.Concat(i2).ToList();
IEnumerable<int> concat = i.Concat(i2).ToList();
// Only those ints from i not in i2.
IEnumerable<int> except = i.Except(i2).ToList();
IEnumerable<int> except = i.Except(i2).ToList();
// Only the unique ints found in i and i2, and order them.
IEnumerable<int> union = i.Union(i2).OrderBy(r => r).ToList();
IEnumerable<int> union = i.Union(i2).OrderBy(r => r).ToList();
// Only those ints that exist in both i and i2.
IEnumerable<int> intersect = i.Intersect(i2).ToList();
IEnumerable<int> intersect = i.Intersect(i2).ToList();
Jason Harper's nerd rant 