Subtivals · A similar program which handles .ass files, amongst other things.
Gaupol · A very useful, complete, and simple subtitle editor, for Linux and Windows. Can be used to convert various formats of subtitles to .srt files.
Aegisub · A free, cross-platform open source tool for creating, modifying, and converting subtitles to .srt files.
Subtitle Edit · Free subtitle editor for Windows supporting many file formats. An online version is also available on the website.