Info-ZIP portable compression/archiver utilities (Zip, UnZip, WiZ, etc.)
Versatile Commodore Emulator
VICE is an emulator collection which emulates the C64, the C64-DTV, the C128, the VIC20, practically all PET models, the PLUS4 and the CBM-II (aka C610). It runs on Unix, MS-DOS, Win32, OS/2, Acorn RISC OS, BeOS, QNX 6.x, Amiga, GP2X or Mac OS X machines.
libcdaudio is an unmaintained but portable library that provides wrapper functions to audio CD drives and CDDB and CDIndex database access. cdcd is a commandline CD player using libcdaudio's CDDB and CDIndex functions.
Laser is a multiple arcade emulator available for several platforms. It uses Marat Fayzullin's Z80 and 6502 CPU emulators. It was originally based on Mike Green's SinVader.
A free implementation of the classic game Dungeon Master from FTL/Interplay for GNU/Linux + SDL. Further implementations: other (POSIX preferred) systems supported by the SDL library.