Subs Factory is a free powerful utility that aims to make the creation, modification and synchronization of subtitles easy.
There's nothing worse when you're watching a film and the subs are either inaccurate or out of sync. Subs Factory features advanced functions that allow you to perform tasks ranging from creation, deletion and modification of subtitles (such as the font type, duration and position) shifting groups of subtitles, splitting and synchronization of subtitle files. The user interface isn't brilliant but you can more or less work out the basics within a few minutes.
The main problem you might find with Subs Factory however is that if you don't have the right codecs installed, you won't be able to even start. Subs Factory can normally open the same formats as QuickTime but it's advisable to install Perian to make sure it can open more formats. The other issue however is that Subs Factory doesn't handle HD films very well because the file size is usually too big.
Subs Factory is a powerful tool to edit subtitles if you've got the right codecs installed.