An mpg123 frontend of sorts, put your mp3s on your LAN.
Makes a set of web pages by scanning your dirs of mp3s, makes them playable by clicking links in web pages you hit with phones, tablets, computers. Built around mpg123 which does the grunt work.
Cross-platform command-line tool for creation of PDF documents from scans/photos of pages in JPEG (.jpg) format and the lightest weight ANSI C library to put multiple JPEG files into one PDF file.
You can add handwritten comments to PDF scans (over original images) with xournal: http://xournal.sourceforge.net/
It supports graphics tablets and saves comments to PDFs as vector data.
libdjvu is a DjVu Viewer for Hanlin V3 written in C. It has infinite levels of zoom in/out, window panning around the page, asynchronous page readahead, multicolumn mode and many other features.