Free software - first grade

LINKS requires: Linux, or Cygwin under Windows, or BSD, etc.
Web browser running in both graphics and text mode. No Flash, CSS, Javascript supported. High speed and graphics display quality
0VERKILL requires: Linux, Windows, or OS/2
Game - ASCII-ART bloody 2D deathmatch
Z80-ASM requires: Linux, DOS
Zilog Z80 processor assembler and monitor (including disassembler)
JET SET WILLY X requires: ZX Spectrum or emulator
Videogame, 8-bit jump-platform

Second grade - commandline skill required

OPTAR requires: Linux
OPTical ARchiver. Prints 200kB of data on A4 page with laser printer, you scan it with a scanner and feed into decoder program. Has an error correction code. Also known as 2D barcode
LUMINAPLEX requires: Linux
Video encoding algorithm family to encode ordinary 4:2:0 and 4:2:2 Y'CbCr with unprecedented colour detail quality
VNCREC requires: Linux
Records a video of a VNC session into a YUV4MPEG2 file, which can be later encoded into usual video formats. Meant for making instructional videos
TWIG requires: Linux
Online photogallery with multi-level previews
SYNTRING requires: C compiler
String synthesizer, low-profile ASCII tablature. Algorithmic synthesis, no Karplus-Strong, supports inharmonicity
REGISTRATOR requires: Linux
Multiple photography noise reduction or resolution quadrupling. 16-bit per colour channel, psychophysical Kalman filter, fog removal
PITCHOTRON requires: Linux
A musical transciption analyzer. Produces a rolling time display video from audio with individual halftones labeled and colour coded.
CASANOVA requires: C compiler
Dating chat hinter. Casanova stores conversation in pairs into a database, like B was replied to A. Then you give it what the other party says and Casanova suggests what you should reply.