Aegisub

Aegisub is a very advanced and easy to use subtitle editor for the ASS, SSA and SRT formats Aegisub is a general-purpose, open-source SubStation Alpha subtitle editor that was created to help in many aspects of subtitle typesetting, timing and editing by correcting flawed features found in the Medusa Subtitling Station and adding many that it lacked.

With many specialized tools to complement the generic, easy to use interface, Aegisub is now the most advanced SSA/ASS subtitle editor in the world.

A comprehensive manual (including step by step tutorials) ensures that anyone can be working with Aegisub in a very short time.

Here are some key features of "Aegisub":
Unicode in UTF-7, UTF-8, UTF-16LE and UTF-16BE encodings
Importing non-Unicode subtitles from over 30 different character encodings, including Shift_JIS, allowing you to resume work on any subtitles regardless of your system locale settings
Loading advanced Substation Alpha (.ass), Substation Alpha (.ssa), Subrip (.srt) and Plain-Text (.txt) (with actor data) subtitle formats
Full tag conversion from SRT to ASS
Multiple Undo levels
Powerful Automation module using the Lua scripting language to create advanced karaoke effects (including per-syllable effects, similar to AssCalc) - or any other kind of subtitles manipulation
Syntax highlighting
Fonts collector to collect all fonts used in a script into a folder
Opening of videos via Avisynth for true preview of subtitles using VSFilter, in any format supported by DirectShow
Aspect ratio overriding for anamorphic videos
Variable Frame Rate videos via timecodes file (v1 and v2), including exporting processed times for hardsubbing
Display of video coordinates under mouse and simple auto positioning support
Audio mode which allows you to open files in any format that your DirectShow can decode, including Wave, mp3, Ogg Vorbis, AAC, AC3 and others
Powerful audio timing and playback mode, including full support for karaoke manipulation
Functions and hotkeys easily synchronizing audio/video and subtitles
Time shifting and manipulation by timestamps or frame numbers (once video or timecodes are loaded)
Advanced find and replace with support for Regular Expressions
Cut/Copy/Paste subtitles via plain-text format, so it can be pasted in any plain-text editor
Translation assistant to easily translate subtitles to another language
Styling assistant to quickly pick styles for each line in subtitles
Styles manager that allows you to categorize subtitles styles, to make working on several projects painless
Keyframe navigation for fast seeking to scene boundaries
Several subtitles manipulation macros, such as making timing continuous or recombining double lines after OCR
Highlighting of subtitles that are visible on current video frame, and a button to select those
Preview subtitle changes imediately by pressing Ctrl+Enter
Shortcut buttons to set text as bold, italic, underline, strikeout, change font or any of the 4 colors
Extensive manual with detailed instructions and tutorials for typesetting and program usage