Not really a tool per-se, Japanese has standardized subset of kanji suitable for computer use. This is the JIS X 0208 (Kanji 1 and Kanji 2 containing 6,879 characters) and JIS X 0213 (Kanji 3 and Kanji 4 containing 11,233 characters). There are several tricks font designer can utilizes, such as reusing radicals (component of a kanji) and combine them. This technique only works for certain type of fonts and still requires hand-tuning.