Sudachi is a type of citrus popular in Japan. The zest and the juice are used in ponzu sauce which is used to flavor many types of dishes. The juice can be distilled to make an alcoholic drink called shochu.
The sudachi fruit are small, green and look similar to a lime but are believed to be a hybrid of a mandarin orange and citron.
The fresh fruit are very hard to find in the U.S. but bottled juice and fresh or frozen zest can be found as well. You can look for it at a Japanese store or you can purchase the juice online at Amazon.com : Sudachi Juice. Sometimes the zest is available too.