You could try puffing it out with steam. Use a pin to poke small holes in the bottom of the hole through the finish. Wet a corner of a clean white rag. Press it to the dent, and apply the tip of a hot clothes iron. It should swell the dent back up to the surface in most cases.
But you will may have to remove the finish first if that first method doesn't work. You can do this by sanding the dented area, or applying a chemical stripper to expose the bare wood. Then apply a wet corner of a white clean rag to the spot, just as you apply the hot tip of a clothes iron. Use distilled water so you don't cause a mineral stain if your water has iron in it. This should puff the wood back to the surface. You should let this dry overnight, and then fine sand and attempt a touch up.
You can also try drops of rubbing alcohol and the tip of a hot new clean soldering iron. Rubbing alcohol contains enough water to puff out the dent (on bare wood) without damaging the finish next to it too much.
But if this doesn't quite do try a colored latex wood filler. I like the Le Pages brand at http://www.LePageproducts.com/ProductCatalog/detail.asp?catid=25&subid=56&
Be sure to use a color just a tad darker than the floor, and maybe mix two colors together. You may have to make deeper holes in the bottom of the dent for this stuff to key in. Fill to the top, but be sure to wipe off any excess from the surrounding wood, else a nasty smear will dry there. Use a wet cloth for this, then a dry cloth to make sure it's all gone. It may take several fills, and drying overnight, to bring it up to the surface.
But now you have to touch up the finish, and most likely re-coat the entire floor. You will need to know your floor finish. If you don't read the second half of the wood floor maintenance article in the Floored News section of this site. Most floor finishes, and especially poly will not blend in well, and you will have to re-coat the whole floor, and even then it will not perfect. I know, because I have to do such repairs for insurance companies, and even I have a hard time making the repair invisible.
But it can be done. Sand well between all coats, and allow each coat to dry 1-2 days. I'm talking about the touch-up coats. Once the area is filled in with at least 2-3 coats of finish, let this cure for about a week, and sand level, with 120 grit sandpaper, then re-coat the whole floor, or just that board involved.
As always your Most humble servant, Joseph, the Wood Floor Doctor.