Work related, but open source: MIT licensed LLSD implementations

LLSD is one of the cooler things to come out of Linden. It's a "semantic file format", intended to be serialization agnostic (you can use binary, XML, or JSON) and it provides strict rules for accessing the data and conversions between data types. There's also a language called LLIDL for describing RESTful interfaces that consume/produce LLSD data.

I've been doing a fair bit of implementation work there recently, including a JavaScript library for LLSD/LLIDL and LLSD JSON support in C++.