Aegisub is a free, cross-platform open source tool for creating and modifying subtitles. Aegisub makes it quick and easy to time subtitles to audio, and features many powerful tools for styling them, including a built-in real-time video preview.

Some highlights of Aegisub:

Simple and intuitive yet powerful interface for editing subtitles

• 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.
• Made 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

• 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.
• 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

• Nearly all 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.
• Automatic live preview of subtitles updated as you type, showing you exactly what the subtitles will look like in the player.
• Full support for variable framerate video and Matroska timecodes. Capable of reading timecodes directly from Matroska files for seamless VFR work.
• Problems with anamorphic video? Overscan? Aegisub has the tools to handle it.

Visual typesetting tools

• 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.
• 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

• 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.
• Support for nearly every audio format in existence, and DirectShow support for if you manage to find one too obscure for libavcodec.

Fully scriptable through the Automation module

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