Sometimes, IEnumerable<T>.Any() is the opposite of what you want...

EqualityExtender.cs

public static class EqualityExtender
    {
        /// <summary>
        /// Sometimes you want to say this.Name = "Bob" or "Fran". Linq lets you do: 
        ///  new[]{"Fran", "Bob"}.Any(n=> n == this.Name);
        /// 
        /// but isn't it nicer to do:
        /// 
        /// this.Name.EqualsAnyOf(new[]{"Bob", "Fran"});
        /// </summary>
        public static bool EqualsAnyOf<T>(this T source, IEnumerable<T> others)
        {
            return others.Any(t => t.Equals(source));
        }
    }

    [TestFixture]
    public class EqualityExtenderTests {
        [TestCase("a")]
        [TestCase("b")]
        [TestCase("z")]
        public void AlphabetIncludesLetters(string letter) {
            Assert.That(letter.EqualsAnyOf(new[] {
                "a", "b", "z"
            }));
        }

        [TestCase(-1)]
        public void PositiveNumbersDontIncludeNegatives(int number) {
            Assert.False(number.EqualsAnyOf(new[] {1, 2, 3}));
        } 
    }