Gnome Subtitles

Gnome Subtitles is an open-source subtitle editor for
the GNOME desktop, based on Mono. It supports the most common text-based subtitle formats, video previewing, timings synchronization and subtitle translation. The current version is 1.6.
Gnome Subtitles is free software released under the GNU General Public License.
Features.
Gnome Subtitles supports popular subtitle formats, such as SubStation Alpha (and also Advanced SubStation Alpha), SubRip and MicroDVD.
It has a WYSIWYG user interface, supporting emphasis (bold, italic and underline styles) and multi-level undo/redo. Gnome subtitles can also perform timing operations, edit subtitle headers and deal with subtitle's encoding automatically.
Video previewing, time shifts, encoding selection and subtitle merge/split have been added in the newer versions.
Similar programs are Aegisub, Jubler, Subtitle Editor, Gaupol, etc.