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 use the Apache 2 license, but check individual project details to confirm.
libbson - A library providing useful routines related to building, parsing, and iterating BSON documents. Used by the cross-platform libmongoc client library that provides a base for MongoDB drivers in higher-level languages.
C# / .NET
Newtonsoft.Json - Included in the popular high-performance JSON framework for .NET (a.k.a. Json.NET).
MongoDB.Bson - Included with the official MongoDB C#/.NET driver.
Metsys.Bson - A BSON serializer/deserializer for .NET.
The MongoDB C++ Driver Library - The official MongoDB C++ driver includes a robust and well tested BSON implementation. You can use just the BSON portion of the driver library if desired.
github.com/jbenet/bson-cpp - A standalone C++ BSON implementation forked from the MongoDB C++ driver, with the non-BSON code pruned away.
github.com/dwight/bson-cxx - Yet another C++ BSON implementation originating from the MongoDB C++ driver. Non-BSON code was eliminated, Endian-awareness was added, and some attempts were made to simplify the interface.
JSON for Modern C++ - A library to make JSON a first-class data type in C++ which can read and write a subset of BSON that can map to JSON.
oatpp-mongo - BSON object-mapper plus MongoDB driver for Oat++ applications.
jsoncons - A C++ header-only library for constructing JSON and JSON-like data formats including BSON, supports encode/decode of BSON byte string.
vibe.data.bson - BSON serialization and value handling.
bson_codec - A flexible serializer and deserializer for BSON documents.
elixir-bson - BSON implementation used by the elixir-mongo MongoDB driver.
elixir-cbson - BSON NIF for Elixir/Erlang language
emongo_bson - Included with the emongo driver for Erlang.
afBson - Implementation created to support the development of the afMongo driver for MongoDB.
Fantomongo driver - A pure Fantom driver for MongoDB.
The BSON library (org.mongodb.bson) - A standalone BSON library, with a new Codec infrastructure that you can use to build high-performance encoders and decoders without requiring an intermediate Map instance.
bson4jackson - A pluggable BSON generator and parser for Jackson JSON processor.
ebson - Extensible BSON encoder/decoder library written in Java with pluggable Java-to-BSON type mappings.
Jongo (org.jongo.bson) - Included in the Jongo driver for MongoDB.
CookJson - Contains JSON and BSON parsers/generators for JSON API for Java (JSR 353).
BSON.jl - A Julia package for working with the Binary JSON serialisation format.
cl-mongo - Common Lisp interface for MongoDB.
bson (nimble package) - A BSON library in pure Nim.
bson - BSON serialization and deserialization library used by the official MongoDB Node.js driver.
Bson.ml - A BSON encoder / decoder in OCaml, built for MongoDB.
MongoDB::BSON - Included with the official Perl driver for MongoDB.
Mango::BSON - Included with Mango, a Pure-Perl non-blocking I/O MongoDB driver.
BSON - Interface included with the official MongoDB PHP driver. Builds on the libbson C library.
prolog-bson - BSON encoder/decoder for Prolog. Used by the Prolog MongoDB community driver prolongo.
PyMongo - official MongoDB driver
bson - Independent BSON codec for Python that doesn't depend on MongoDB.
py-bson-rpc - A Python library for JSON-RPC 2.0 and BSON-RPC on sockets (TCP and TCP+TLS).
bson (with optional C extension) - A full featured BSON specification implementation used by the official MongoDB Ruby driver.
bson - Encoding and decoding support for BSON built using the Serde framework.
MongoDB Scala driver - Official MongoDB driver.
ReactiveMongo - Reactive Scala Driver for MongoDB.
Mongo-BSON - Part of the MongoTalk driver for MongoDB.
mlmongo - Standard ML driver for MongoDB.
BSONSerialization - BSON Serialization in native Swift 4.
BSON - Native Swift library for BSON.
bsontools - Command line utilities for manipulating BSON files and for translating them to and from other formats such as CSV, JSON, and XML.
MongoDB - A distributed document-oriented database using BSON as both the network and on-disk representation of documents.
ModeShape - A hierarchical JCR data store using BSON to serialize the data.
bsonsearch - BSON matching engine to perform offline document comparisons without a MongoDB server. C dynamic library with Python wrapper.
If you know of other BSON implementations or projects using BSON, please add them.