Most interior walls are trimmed along the floor with baseboard molding. At each outside wall corner, the molding is typically cut at a 45-degree angle to form a miter joint.
Over time, these joints have a tendency to open up, resulting in an unsightly gap. In many cases, you can close the joint in a matter of minutes using nothing more than a round-shafted screwdriver.
Rub the screwdriver shaft back and forth across the joint, applying just enough pressure to compress the wood fibers. The idea is to slightly round over the corner and conceal the gap.
This technique won't work on very large gaps; you'll have to fill those with wood putty or cut a new piece of molding to fit.