A standalone app that floats on top of any video you're watching, JustSubsPlayer is a simple way to add subtitles to everything from DVDs to browser videos.
It's now really easy to find subtitles (usually in .srt format) online for almost any program or movie you want to watch thanks to a huge community of people worldwide creating subtitles. Many media apps like VLC player support subtitle file playback, but it can be awkward to correct synchronization if you have mismatched files.
JustSubsPlayer allows you to run subtitle files over any video, as it is a separate app that can be placed over whatever you are watching. Using JustSubsPlayer's interface make it really easy to sync your subtitles in seconds.
You can resize the subtitles to suit you, and pause and search them too. JustSubsPlayer is a basic little app, and there are no customization options. This means you can't change the font or color of your subs - they always appear as a black band with white text. So, you can always read them, but some people might prefer a transparent background option that interferes less with your picture.
JustSubsPlayer is really easy to use, lightweight and stable. It is a bit basic, but does it's job well and has probably the simplest controls for syncing subtitles with movie files.