Columbus Cutlery (358 Columbus, 362-1342) has for decades been San Francisco chefs' favorite place to get their knives sharpened. The service is professional, turnaround is usually overnight, and prices are very reasonable: $4 up for blades up to 8", $5 for 10-12", and $6 beyond that. They also do scissors and pocket knives, can repair even badly damaged knives, and stock replacement toothpicks and tweezers for Swiss Army knives.
On top of that, the tiny shop offers one of the biggest selections of knives and scissors in the area. The following photos (click to enlarge) show only about half the items in stock.
The store is open 9:30am-5pm, closed Wednesdays and Sundays.