From Aegisub Project:
Aegisub is a free, cross-platform, and open source tool for creating and modifying subtitles. Aegisub features a built-in real-time video preview of the subtitles, along with tools for timing them to audio and manipulating their appearance. Create new subtitle or edit existing subtitle. You can export the subtitle with filter options transform framerate and fix styles. You can also include font or graphical attachments.