public static string ToString<T>(this IEnumerable<T> source, string delimiter)
        {
            return string.Join(delimiter, source);
        }

That will throw up an exception if ‘source’ is null. That’s fine for me, since I don’t want that extension from hiding something that might be wrong elsewhere (i.e. why am I trying to join a null list). If you want to make this work fine and dandy for a null source, you can just use the null coalescing operator and provide an empty source:

        public static string ToString<T>(this IEnumerable<T> source, string delimiter)
        {
            return string.Join(delimiter, source ?? Enumerable.Empty<T>());
        }

Now why do I think this is a better solution that the string builder option? Well, if you whack open Reflector, you’ll notice that the Join method uses a StringBuilder object and builds up from there. So since it’s already built straight into the framework, why reinvent the wheel? Of course, the Join method with an IEnumerable as an argument is only available on .NET 4.0. Anything prior to .NET 4.0 and you’ll have to pass in a string array (at which point the string builder method would be useful).

Another point to notice from the documentation is that the Join method will set the separator to String.Empty, if you pass it a null. The same applies to any element in the list that is null – it is simply replaced by another empty string.

public static IEnumerable<IndexedType<T>> Index<T>(this IEnumerable<T> source)
{
	return source.Select((x, i) => new IndexedType<T>() { Item = x, Index = i });
}

And the class returned:

public class IndexedType<T>
{
	public T Item { get; set; }
	public int Index { get; set; }
}

This could be extended to include values like if the index value is even, N-Indexed values, etc but I leave that up to the imagination of the programmer.

I don’t like Enumerable.Range() for the sole reason you have to give a start index and a count. So I made up my own. This is an extension for an integer type and will allow you to specify two values that you want a range for

public static IEnumerable<int> RangeTo(this int startValue, int endValue, int step)
{
	return (endValue < startValue) ?
		Enumerable.Range(endValue, startValue - endValue + 1).Reverse() :
		Enumerable.Range(startValue, endValue - startValue + 1);
}

This will also allow for reverse lists – see how to use it below:

1.RangeTo(10);  // {1,2,3,4,5,6,7,8,9,10}
5.RangeTo(10);  // {5,6,7,8,9,10}
10.RangeTo(5);  // {10,9,8,7,6,5}
3.RangeTo(3);   // {3}

int x = 15;
int y = 17;
x.RangeTo(10);  // {15,14,13,12,11,10}
y.RangeTo(x);  // {17,16,15}
x.RangeTo(y);  // {15,16,17}
13.RangeTo(x); // {13,14,15}

public static IEnumerable<T> Random<T>(this IEnumerable<T> source)
{
	return source.OrderBy(x => Guid.NewGuid());
}

For example, to return a random set of 5 items from the collection:

var randomSet = myEnumerable.Random().Take(5);

If anyone knows of a better way, let me know