Aegisub was originally created as a tool to make typesetting, particularly in anime fansubs, a less painful experience. At the time of the start of the project, many other programs that supported the Advanced Substation Alpha format lacked (and in many cases, still lack; development on several competing programs have since been dropped for various reasons completely unrelated to Aegisub) many vital functions, or were too buggy and/or unreliable to be really useful.

Since then, Aegisub has grown into a fully fledged, highly customizable subtitle editor. It features a lot of convenient tools to help you with timing, typesetting, editing and translating subtitles, as well as a powerful scripting environment called Automation (originally mostly intended for creating karaoke effects, Automation can now be used much else, including creating macros and various other convenient tools).

Aegisub Features:
1. Unicode in UTF-7, UTF-8, UTF-16LE and UTF-16BE encodings
2. 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
3. Loading advanced Substation Alpha (.ass), Substation Alpha (.ssa), Subrip (.srt) and Plain-Text (.txt) (with actor data) subtitle formats
4. Full tag conversion from SRT to ASS
5. Multiple Undo levels
6. 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
7. Syntax highlighting
8. Fonts collector to collect all fonts used in a script into a folder
9. Opening of videos via Avisynth for true preview of subtitles using VSFilter, in any format supported by DirectShow
10. Aspect ratio overriding for anamorphic videos
11. Variable Frame Rate videos via timecodes file (v1 and v2), including exporting processed times for hardsubbing
12. Display of video coordinates under mouse and simple auto positioning support
13. 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
14. Powerful audio timing and playback mode, including full support for karaoke manipulation
15. Functions and hotkeys easily synchronizing audio/video and subtitles
16. Time shifting and manipulation by timestamps or frame numbers (once video or timecodes are loaded)
17. Advanced find and replace with support for Regular Expressions
18. Cut/Copy/Paste subtitles via plain-text format, so it can be pasted in any plain-text editor
19. Translation assistant to easily translate subtitles to another language
20. Styling assistant to quickly pick styles for each line in subtitles
21. Styles manager that allows you to categorize subtitles styles, to make working on several projects painless
22. Keyframe navigation for fast seeking to scene boundaries
23. Several subtitles manipulation macros, such as making timing continuous or recombining double lines after OCR
24. Highlighting of subtitles that are visible on current video frame, and a button to select those
25. Preview subtitle changes imediately by pressing Ctrl+Enter
26. Shortcut buttons to set text as bold, italic, underline, strikeout, change font or any of the 4 colors
27. Extensive manual with detailed instructions and tutorials for typesetting and program usage

Some highlights of Aegisub:
Simple and intuitive yet powerful interface for editing subtitles
1. With help from such features as spell checking/thesaurus, a translation assistant, call tips for override tags, syntax highlighting and various macros, translating, editing and quality assuring subtitles is now easier than ever.
2. Did a mistake? Had a system crash? Don't worry, Aegisub saves your .ass with features like multi-level undo/redo, auto saving, backups and emergency recovery saving in case of system or program crashes.

Support for many formats and character sets
1. Aegisub doesn't lock you into using any specific format; importing and exporting from and to several different formats (ranging from MPEG-4 Timed Text to MicroDVD) is supported.
2. Multiple language subtitles? Legacy programs without Unicode support? No problem, Aegisub has full support for both Unicode and most common legacy encodings; both reading and writing.

Powerful video mode
1. Most video common video formats are supported out of the box, but if that isn't enough, Aegisub can also use your system's DirectShow environment to open any format you can play in Windows Media Player.
2. Full support for variable framerate video and Matroska timecodes. Capable of reading timecodes directly from Matroska files for seamless VFR work.
3. Problems with anamorphic video? Overscan? Aegisub has the tools to handle it.

Visual typesetting tools
1. Typesetting on easy mode! No need to manually edit override tags; drag, rotate or clip the subtitles directly on the video frame using your mouse.
2. Want to re-use typesetting on a video of different resolution or aspect ratio? Can't figure out what color that sign is using? Aegisub has the tools to help you deal with it.

Intuitive and customizable audio timing mode
1. Time dialogue or karaoke in any way you want with the heavily customizable audio interface; less painful than ever! Use features such as the spectrum analyzer mode or the automatic timing post-processor for even more efficient timing.
2. Support for any audio format your DirectShow environment knows how to decode.

Fully scriptable through the Automation module
1. Is all the above not enough? Extend the program with your own macros or export filters using scripting in Lua or Perl.
2. Comes with several ready-made scripts for quick and easy generation of your very own karaoke effects.

The license of this software is Freeware, you can free download and free use this audio utility software.