Did you forget to clean the finish from your paintbrush after your last project? Don't throw it away! There's still a good chance it can be salvaged. Soak the brush in a small container of the appropriate solvent (see can for instructions). As a general rule of thumb, you can use alcohol to loosen up shellac and lacquer thinner for lacquer. For water based or oil finishes try using paint and varnish removers such as acetone, xylene, or tolulene.
After the brush has soaked for a while, work the solvent into the bristles with your hands (wear gloves) and a brush comb (available at most paint stores) or an old fork.
Repeat this process until most of the old finish has been removed and then soak the brush in a strong solution of TSP (trisodium phospate). You can find TSP at most hardware and paint stores.
In many cases these steps will help reclaim the brush. Be sure to wrap the bristles in brown paper or a heavy paper towel to help maintain their shape.