Filling a roundish hole in a plank is a not-uncommon requirement: knot holes, or reclaimed wood accompanied by cuts for plumbing and heating, can present boards that need something beyond wood filler - they need a plug. Here are four tools we've run across, and a bit about a project involving hole plugging.

From Rockler, here's a set of plug cutters engineered to give precision-ground edges and a flute design that cuts precise holes that won't require sanding. Suitable for both soft and hardwoods, its plug diameter is slightly tapered from one size to .025" oversized. Available in 1/4", 3/8" and 1/2" plug sizes. A 1/4" hex shank on all the cutters fits quick-change chucks (sold separately).

The Rockler Plug Cutter is intended for use on a drill press or similar stationary machine. Hex shank can be used in keyed, keyless and quick-change chucks. Features:Made from heat treated steel for extended tool lifeFlute design for smooth, clean, precise cutting action in any type of wood. It makes wooden plugs with chamfered/tapered shapes for easy insertion into holes, a snug fit and minimal glue line. Hex shank can be used in keyed, keyless, and quick-change chucks.

Grizzly sells a six-piece plug cutting set, in a nice pine box - so you can readily size the plug you're cutting to the hole you want to fill.
Cut plugs and long tenons up to 2-1/8" with this 6-pc. Deep Plug Cutter Set. Diameter sizes include 3/8", 1/2", 5/8", 3/4" 7/8" and 1". Each cutter is protected in a fitted wooden case. Shank size is 1/2".

Alex at Old Town Home has been working on restoring a floor with heartwood pine. But the flooring is loaded with holes from a defunct heating system. Here's what he's been doing to even out the hole, and to cut a plug that fits:

I make sure the hole is exactly the right size. I like using my spade bits to open and round them a bit, though a hole saw is probably a better choice as it won't bounce around as much if the hole is already a very odd size.

The main issue you face is that the hole is already there, so you need to use something to guide the bit you're using to ensure the hole is clean. The easiest way to do this is with a scrap piece of wood that has the right-sized hole already cut.

You center the scrap hole template over your existing hole, insert the bit into the hole, and then start the drill above where you want to drill it. If you can do it steadily enough, you'll have a rounded out hole that's exactly the right size for the plug you'll cut.

The General Tools plug cutter set produces accurate plugs for covering screw holes in decking, flooring and fine furniture. Plug cutters are great for creating a decorative effect in any woodworking project and can also be used to repair nicks, scratches and dents. Each cutter is made from hardened alloy steel and will fit in both hand-held power drills and drill presses.

General Tool plug cutters

AskWoodMan shows how to cut a lot a plugs quickly and accurately on a drill press to be used to plug screw holes in Baltic birch plywood. One nice trick: he scribes pencil lines parallel to the wood grain, so when the plugs are ut loose from the boar, it's easy to identify and match grain direction when dropping the plug into a hole.