Too many years ago, I begun my computer programming experience with a ZX Spectum. Since those times, I've had a growing sensation of nostalgia about the early ages of home computing and nowadays I'm still playing with that old and useless stuff! :-)
z88dk is a C cross-compiler; more precisely, it's an advanced set of tools allowing the production of complex programs for Z80 based computers in either C or assembly language. It can be compiled & installed without any particular effort on Linux/Unix, while on Windows I suggest you to download the zip file form the Nightly snapshots and run z88dk_prompt.bat (in the past I made a setup executable but nowadays it's not needed anymore).
This program is a simple sprite editor (SpEdito = sprite editor) for the z88dk compiler. Sprites can be saved to files and than re-loaded; SpEdito can finally generate C code for the sprites, allowing you to use them inside the z88dk monochromatic game library (maybe the support for the Sprite Library SP1 will be added in future via external module). This old example (4 KiB) is a very simple ZX Spectrum application that uses a sprite generated with SpEdito (the z88dk monochromatic game library works with a lot of computers; the ZX Spectrum is just one of them).
SpEdito 1.0 - binary, source code plus two external modules: Small Preview and ASM Generator
This is a tic tac toe game against the computer: the program shows you the Speccy learning from its own mistakes (when it looses a match, it saves its moves and tries not to repeat them, to avoid loosing again). At the beginning of the game, the only thing the computer knows is that it has to move where there are 2 other equal Xs or Os in the same row. Obviously this is not enough to play well; ...game by game it will learn not to loose at all.
A.I. Tic Tac Toe compiled for ZX Spectrum + Windows x86 32-bit console executable + source code