I googled it last year and came up with some good resources, but no sample essays. The best advice I can give is take the broader impacts very seriously in your essays. All of them.
When I was working on my proposal last year, I used this resource from UC Santa Cruz:
UC Santa Cruz - Division of Graduate Studies - Student Affairs - NSF Graduate Research Fellowship Information
It has essays from someone who was rejected the first year but won the award the second time she applied. This is especially useful because her qualifications really didn't change, just her essays, so you can really see how you need to present yourself.
I found a useful link for an economics NSF essay that won the fellowship:
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