If you are having a problem with water running down the inside of your chimney and accumulating in the fireplace, here's a post for you.
The problem probably stems from a bad crown on top of a masonry chimney. Usually the crown is a top made from concrete that is angled to shed water (it overhangs the brick portion to keep drips from running down the chimney sides). This structure surrounds but does not encase the clay flues. The problem of water leaking usually comes in older homes. Before the mid-1980s, chimney crowns were simply sloped washes put together with leftover mortar. These crowns can crack and deteriorate, which leaves gaps around the flues. This is what causes the leaking. Some people believe adding chimney caps will solve the problem, but if you have a bad crown that might not be enough.
The solution? If you are comfortable going up on your roof, head up and check the crown. Use polyurethane to caulk any cracks or gaps you find. If you're not comfortable on the roof, call a pro. Check in the yellow pages under "Chimney Builders and Repair" or even under "Chimney Cleaning" for a weep who can handle the problem.
Source: "Water Down the Chimney" from the Dec/Jan 2006 issues of the Family Handyman