Vista / Win2k / Win7 / WinXP
It was originally created as a typesetting tool to replace Medusa. It is open source software and free for any use. Aegisub natively works with the Advanced SubStation Alpha format (aptly abbreviated ASS) which allows for many advanced effects in the subtitles, apart from just basic timed text. Aegisub's goal is to support using these advanced functions with ease.
aegisub ass advanced substation alpha srt sub
1License and operating system information is based on latest version of the software.