Implementations of BSON exist for many different programming languages. Some implementations are currently embedded within MongoDB drivers, since MongoDB was the first large project to make use of BSON. Over time those libraries will be made more stand-alone, but they should be usable independently of MongoDB in their current state.
Most of these libraries are Apache 2 license, but that depends who wrote it, so check the individual project's details.
Dart: bson_codec (standalone)
J2ME (work in progress)
Swift (OpenKitten BSON)
If you know of other BSON implementations or projects using BSON, please add them.