I am often asked to provide sample recommendation letters. In fact, you probably found this blog entry because that is exactly what you were looking for! What I've found is that those that want samples are really looking for a better understanding of how ideal recommendation letter answers should be structured, the level of detail required, and what kind of assessment of the applicant is provided in them. For this, rather than samples, all that is needed is good advice.
Here are the factors that, to me, make a recommendation letter impactful.
1. They include specific and detailed examples.
A good recommendation letter offers clear and specific examples. Ae you a great leader? If your supervisor agrees, than they may write this in their letter. However, it is too easy just to write "so and so is a great leader". Instead, he/she should be able to derail a specific episode that shows you are a great leader. What did you do? What did you not do? How did you do it? These details should be included, and from the perspective of the referee. This last detail is important, and is picked up in the next point.
2. They include opinions as to how you have developed and in what ways.
It's not enough for a recommendation letter to relay the story behind one of your great successes. Rather, a good recommendation letter should (a) relay that story from the point-of-view of the referee, and then (b) he/she should detail the strengths or skills that you demonstrated or developed over the course of that episode. This, in total, offers a detailed and comprehensive assessment of you.
3. They have insightful and interesting strengths and weaknesses.
I think the strengths part here is obvious. If the writer of your recommendation knows you well, then it stands that they should also know your weaknesses. No one's perfect right? I recommendation letter with no weaknesses, or one with uninteresting weaknesses (time management is simply not insightful enough to stand out.
If your recommendation letter has all three of these things going for it, then chances are it will stand out from the crowd, because it will be insightful and memorable. That's what makes a recommendation letter good.