I picked up a Casio FX-3600p calculator on eBay recently.

I took a calculus class in college from a professor who inspired me. He'd grown up in a world where people did calculations by hand, and he saw calculators as tools that would free mathmeticians from grunt work and let them do the theoretical work, the dreaming. He required students to spend a little more and get programmable calculators. We spent the semester getting to know our calculators and getting to use them to do all of our heavy lifting, so to speak.

Before that class, I mostly used computers to play games. After that class, I saw calculators and computers as tools to do the repetitive, error-prone work and started programming in earnest.

I was a poor college student, so I picked the Casio - it was the cheapest option available. Other kids splurged on the HP 41CV or HP 71 calculators, which I would have loved to have. I have an HP 41 emulator on my phone, that's the closest I'll come to one.

The FX-3600p is a lightweight compared to today's graphing calculators - it could store two programs in memory and each was limited to (I think) 38 steps. That was enough to store equations and let you run through several iterations to graph it, saving a ton of time.

It does have its charms, though. The calculator I received arrived in great shape, and the original lithium CR2025 battery is still going strong.