We've all heard the old adage that there's nothing more dangerous in the kitchen than a dull knife. A sharp knife cuts cleanly through things. A dull knife forces you to use more pressure, then may bounce off the surface of the onion you're trying to slice and onto another surface, such as--for instance--your hand. Ouch!
Our favorite knife sharpener in town is Henry Westpfal, at 105 West 30th Street, between 6th and 7th Avenues. Broadway Panhandler on 8th Street between Broadway and University also sharpens knives, as does Samurai Sharpening Service in the Chelsea Market on Wednesdays and Saturdays. We've also seen a knife sharpening truck on the streets of Park Slope. You can do it yourself, too; eGullet has a guide here, as does knifecenter here.
On March 31st, Broadway Panhandler is having a charity knife sharpening event. For $10, you can have up to three knives sharpened while you wait and the proceeds will be donated to City Harvest, and you can hone up on your knife skills noon to 4:30 with free demos from Wusthof.