History
Naoki Takahashi's NScripter is a popular Japanese game engine used for both commercial and free visual novels. It attained popularity due to its liberal terms of use and relative simplicity. However, it is closed source software and only available for Windows.
A number of cross-platform clones exist, and Ogapee's ONScripter is the most popular of these. Due to the ease with which it can be modified to support languages other than Japanese, ONScripter has been adopted by the visual novel localisation community as the engine of choice for translated NScripter games; the patch to support English even made it into the main ONScripter source code.
Nevertheless, the needs of the English visual novel translation frequently differ from Japanese ONScripter users, so we are maintaining our own branch of the ONScripter code to allow us to focus on the features that are most relevant to playing translated NScripter titles.
ONScripter-EN
UncleMion is the current primary developer for ONScripter-EN, and Roto is responsible for most Mac OS X-specific features and support.
More information about ONScripter-EN can be found at UncleMion's ONScripter Corner. We are also often available in #denpa on irc.synirc.net.