I got this hint from macosxhints somewhere.

Say you downloaded movies from scene, sub included, usually it’s on sub + idx format. Many media player don’t work well with this, or you just want plain srt files. What you need is D-Subtitler, OCR (Optical character recognition) app, it scan an image and encode it into text. This is an old released app. Sometimes the result is just muddy.

Way to do it:

• Change extension *.sub to *.vob
• Drag and drop to D-Subtitler. When it says it’s waiting, click on the green circle (I always thought I have to wait for something).
• So far all subs I got only work with Choice of gray levels: 255,255,0,255
• Use TextEdit to scan any error (Edit > Spelling and Grammar > Show Spelling and Grammar ) ⌘:
• You can use more complex and serious app like MovCaptioner. This is actually used for captioning.

[Update] Here‘s another app, Avidemux.