My first computer. I had already taken a programming course at school. I was working on a project that required some repetitive calculations (designing a loudspeaker using textbook formulas). The ZX81 went on sale at K-Mart for 40 bucks, and I rode my bike out there and got one.

I couldn't get the cassette storage to work, so for several days, I begged my family to not unplug it. I remember one amusing thing about its architecture, which was that it used the CPU for video generation, and did all of its execution of BASIC programs during the vertical blanking interval.