libdvdcss is a cross-platform library for transparent DVD device access with on-the-fly CSS decryption. It currently runs under Linux, FreeBSD, NetBSD, OpenBSD, BSD/OS, Solaris, BeOS, Win95/Win98, Win2k/WinXP, MacOS X, HP-UX, QNX, and OS/2. It is used by libdvdread and most DVD players such as VLC because of its portability and because, unlike similar libraries, it does not require your DVD drive to be region locked.
libelf lets you read, modify or create ELF files in an architecture-independent way. The library takes care of size and endian issues. For example, you can process a file for SPARC processors on an Intel-based system. This library is a clean-room rewrite of the System V Release 4 library, and is supposed to be source code compatible with it. It was meant primarily for porting SVR4 applications to other operating systems, but can also be used as the basis for new applications (and as a light-weight alternative to libbfd).
libencio is a library providing a stdio-like interface for reading and writing of encrypted files (in MCrypt format only for now). Additionally, it provides full support for fseek()-like random read access of encrypted data. This allows one to operate on encrypted files as if they were ordinary, cleartext files. It could be used to provide MUAs with a layer to transparently handle encrypted attachments, as a backend to ffmpeg or mplayer to directly play encrypted files, or in combination with tar for encrypted backups. It uses libmcrypt and libmhash for encryption and hashing algorithms.
liberror is a library for C and C++ that addresses the allegedly simply task of printing messages. It is essentially a feature-rich substitute for fprintf. Features include colours via ANSI mark-up, multiple error streams, report files, automatic error number assignment, filtering by number, subsystems, and groups, buffering with manual filtering and re-issuing, callbacks, error counters, time stamping, and message styles.