Apparently because of El Niño Costa Rica will be sunny or less rainy in October and November, 2009. You can take advantage of the low season rates with the summer weather. It's a good time to think about travel to Costa Rica, especially the beaches.
The dry season in Costa Rica runs from the end of November to the end of March. The rainy season generally runs April-mid November. This applies to most of Costa Rica, but not the Caribbean side. Two of the rainiest months are November & December, while September & October are their driest. Most of the rain on Caribbean side comes at night and is more spread throughout the year. Unlike Pacific, there is no really defined dry or wet season. As far as actual volumes of rain, many places on the Pacific actually have more - it is just that it rains in a shorter time period.
There is a quick "mouse-over" guide here to give you an idea of a rain overview of the whole country. http://www.costaricaguides.com/Climat...
The rainy season is becoming increasingly just as popular as dry season as tourists learn that on the Pacific side it usually rains in the evenings and even in rainy season the days are mostly filled with sunshine. The rain also helps keep the dust down on the mostly unpaved roads. There is also a very popular period from mid July to mid-August that is referred to as "little summer" when it usually doesn't rain. The rainiest month on the Pacific side is usually October when it can rain all day long.