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. The last version of VobSub was version 2.23, after which the development of VobSub ceased. VSFilter was a part of the "guliverkli" project on the SourceForge web site. The "guliverkli" project also includes the ability to extract subtitles from a DVD via the "vobsub ripper" program. However, development of "guliverkli" ceased in 2005 with version 2.37. In 2007, it became a part of the "Guliverkli2" project, now known as DirectVobSub, starting with version 2.38. But till September, 2012, the last guliverkli2 commitment was on April 10, 2011. Then, the MPC-HC project took over the sources. Under that project, DirectVobSub was very briefly in active development again, but MPC-HC shut down in 2017 due to a lack of developers.
DirectVobSub is also capable of extracting subtitles from a DVD without ripping it to a file first. 
"Vobsub rippers" are also available for the same effect. Note also the guliverkli project has a related project VSrip.