I'm familiar with Java, JavaScript and C. For most of the time, I'm working with Java.
I'm interested in all kinds of basic libraries, like lossless data compression, data serialization and data extraction.
I'm helping out maintaining Apache Commons Compress, which is an excellent Java library that deals with all kinds of compressions.
I'm also working in Apache Tika, which is a great toolkit helping you with file type detection and content extractor.