Iterators in C# are probably one of the least understood and most wonderful features the language has. They were added in C# 2.0 and are the magic that gives LINQ its Deferred Execution goodness. Anyway, I was needing an arbitrary length list of Guids today (although, with the recent shortage, perhaps ScottGuIDs would have been a better choice) and used this cute little few-liner to get what I wanted:

public IEnumerable<Guid> Guids
{
    get
    {
        while (true)
        {
            yield return Guid.NewGuid();
        }
    }
}

From there you can easily get yourself a list of, say, 5 Guids using LINQ’s Take Extension Method:

var myGuids = this.Guids.Take(5);
Obviously, this made me want to generate some more interesting sequences, like Fibonnaci’s:
public IEnumerable<int> Fibonnaci
{
    get
    {
        int first = 0;
        yield return first;

        int second = 1;
        yield return second;

        while (true)
        {
            int next = first + second;
            first = second;
            second = next;

            yield return next;
        }
    }
}

Pretty terse, right?