Kevin Hancock: Use a plug cutter and cut the plug from the same face of the board.

John Brock: Find a scrap or closely matching scrap of wood. Cut a plug with the proper grain orientation. Drill out the hole to match the plug. Glue the plug into the hole using a glue that will not cause later finishing problems (like hide glue). After it dries, sand or scrape the plugged area smooth. If you are careful about the grain orientation, the repair will be nearly invisible. If the repair shows, pick up a copy of Michael Dresdner's "The New Wood Finishing Book" and learn how to draw and blend in the missing grain lines before final finishing.

Greg Williams: The easiest way is to use an epoxy stick. The traditional way is to drill a larger hole that will accept a press fitted glued dowel of a similar wood. Apply glue, drive the dowel in, allow the glue to dry, cut the dowel flush, and redrill the hole to the proper size and location.