CCExtractor is an Open Source closed caption tool.

It supports American TV captions (CEA-608 is well supported, and CEA-708 is starting to look good) and Teletext based European subtitles.

CCExtractor integration with other tools:

Integrating CCExtractor in a larger process is possible. A couple of tools already call CCExtractor as part their video process - this way they get subtitle support for free.
Starting in 0.52, CCExtractor is very front-end friendly. Front-ends can easily get real-time status information. The GUI source code is provided and can be used for reference.
Any tool, commercial or not, is specifically allowed to use CCExtractor for any use the authors seem fit. So if your favorite video tools still lack a captioning tool, feel free to send the authors an email.