OpenCC is a project aims at Traditional Chinese to Simplified Chinese Conversion and vice versa. it provides high-quality word databases, conversion tools and libraries(libopencc) on Windows/OSX/Linux systems.
It has been selected by many mainstream open source Chinese IME for such purpose, eg: ibus-pinyin/fcitx/libgooglepinyin and etc.
It can strictly distiguish one to many word conversion situation, be compatible of old and abnormal words and dynamical conversion. It supports zh_CN/zh_TW/zh_HK abnormal words' intersectional conversions, results optimization by using interpretations split and minimum split principle, word database extensions, and can be called using many programming languages, or by CLI/GUI.
code.google.com/p/opencc