Commodore 64. With the infamous 1541 floppy drive, and the world's loudest and slowest dot-matrix printer. It was originally a family present, but I ended up spending the whole of that summer in traction, mostly in an isolation room (on account of being the only one on the children's ward who hadn't come down with the mumps), so my parents set it up for me on a couple of those hospital table things so I would have something to do.
After a while I got bored with the handful of games we had, so started to investigate the programming manuals. The hospital teacher considered this sufficiently educational to leave me to it, which was infinitely preferable to her tedious written work. I was only 7, with limited technical vocabulary or understanding of algebra and other useful concepts, but I patiently cargo-cult programmed my way into making it produce *my* random beeps and graphics, as well as printing "$little_brother smells of poo" hundreds of times on fanfold paper. Quality stuff.
While I never became a real programmer, it instilled a fascination with the technology that, for better or worse, got me where I am today.