The good old Sinclair ZX81, one of the first truely affordable home computers that was, for a generation the first introduction to programming. It replaced the (extraordinary for the time) ZX80 and surpassed it in just about every respect.
As for this site, it contains all the source, tools and info you need to write HiRes games on a real ZX81...
The real thing
Is it worth getting hold of a real ZX81 ?
Getting as close as you can
Firing up ZX81 programs on your laptop
The how and the why
Toolkits for developing HiRes applications on a real ZX81