Aegisub is an advanced and portable subtitle editor for Windows, and UNIX-like systems, such as Linux, Mac OS X and BSD. 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.
Download: Aegisub 2.1.8