I've become interested in the different subtitle file formats, and here is a list of the ones I've come into contact with:

Name Extension(s) Format Comments
Advanced SubStation Alpha .ass Text Widely used by fans who do subtitles
Synchronized Accessible Media Interchange .smi Text Microsoft format now happily rare
PGS subtitle format .sup Binary Subtitle format used on Blu-Ray movie
SubRip Text .srt Text Basic but very useful subtitle file format
VobSub format .idx/.sub Text / Binary Subtitle format used on DVD movies