One of my all time favorite calculators is the HP 16C, an RPN calculator for programmers with hex, binary, octal support. While it doesn't run Linux, the HP Prime G2 does. This graphing calculator has shocking specs for its category: an i.

MX 6 Ultralite ARM Cortex A7 CPU, 256 MB DDR3 RAM, and 512 MB internal storage - closer to a small embedded Linux machine than a traditional calculator. The G2 runs Free RTOS, but I've updated an old unsupported Linux port (4.14 kernel) with enhancements like better keyboard support and a custom driver for the newer Ilitek ILI211X touchscreen controller. The updated port includes USB serial console access, X11 server, Doom at great framerate, and a C compiler for on-device development.

My goal was to maintain full administrative control over devices I own, following a personal rule that devices must allow root access. Most calculators are locked down due to cheating concerns, but HP never implemented assembly support on the Prime, making it uniquely open. The TI Nspire CX (II) CAS is the only other calculator I found with a Linux port, running Arch Linux on a 396MHz ARM CPU with 64MB RAM.