If you use a iPhone or iPod, then ALLConverter To iPhone is a useful application to that will merge external subtitles with video/ audio to make friendly multimedia files for the iPhone or iPod playback. This tool can be used to convert all known formats with subtitles. All you need is to pick a video file with the format like mkv/DivX/Xvid/flv and external subtitles with formats smi/srt/sub/txt/mpl2 to make quick conversion.
The converter comes with built in profiles for iPhone/iPod so no extra adjustments are required. The process can be terminated any time and the file with subtitles will be created.
The interface of application resembles the iPhone and the settings are integrated to the interface.