DirectVobSub
DirectVobSub (formerly known as VSFilter) is a software add-on for Microsoft Windows (a DirectShow filter) that is able to read external subtitle files and superimposes them on a playing video file. DirectVobSub/VSFilter were formerly part of a whole application known as VobSub which was also able to extract subtitles from DVD Video and create text-based subtitles, without ripping the DVD to a file first.