Although sake is often described as "rice wine" to Westerners, sake is actually a fermented-grain beverage akin to beer, and unlike wine made from grapes it does not age well. So the winter months, when shinshu (freshly made new sake) is available, are the best time to enjoy this quintessentially Japanese beverage.
A byproduct of the sake-making process is sakekasu, the lees left behind after the liquid is expressed from the fermented rice. While you can buy vacuum-packed or frozen sakekasu year round these days at any supermarket in Japan, the fresh sakekasu you can buy from sake makers is the best, and full of complex flavors.
Before I talk about how to use sakekasu in cooking, though, I'd like to address a question that I'm asked a lot: Why are sake and its sweeter cousin, mirin, used so much in Japanese cooking? While wine, beer and other alcoholic beverages are used in European cooking, they aren't nearly as ubiquitous. Sake and mirin are required basic ingredients in any Japanese kitchen, along with soy sauce, miso, salt, sugar, rice vinegar and dashi stock.