Aegisub
Aegisub is a multi-plattform (Win32, OSX and Linux) subtitling suite focused on the fansub community.It supports many subtitle formats but it's native format is Advanced SubStation alpha.
It integrates many tools for translating, typesetting and all you need to make good subs. It also comes with a script engine based on LUA to create powerfull effects.
There are no official OSX or linux binaries yet but it's possible to get it building your own (using xcode, project files provided) from the source code located in the SVN or to download user-built binaries. Official binares are supposed to come with next release(1.11)



2 Opinions:
It's multi-plattform (Win32, OSX, Linux) but there aren't yet official binaries for mac, you can build it with xcode with the sources from it's SVN or download user-made binaries (I don't remember where to get them, I use to build my own)
It's for Windows!!