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fxIP is a TCP/IP / IPv4-capable network stack for Casio fx-series (9750GII, 9860G) graphical calculators.
It connects to a network via the 2.5mm serial (5V UART) port on the calculators and
uses SLIP (similar to PPP) encapsulation to transmit IP packets over serial.

Example applications for fxIP include a graphical IRC client, a webserver and a small TCP hello world program. Other application can be built and tested against uIP/Protosockets. It (currently) requires calculators with a SuperH SH4a CPU, needs quite a bit of SRAM (32+ KiB). The older models are currently not supported (less RAM, different UART register offsets), but support is should be possible with a bit of careful tweaking.

Further info:
Firmware (uIP port, IRC client)
Chatting on IRC with fxIP (Twitter thread)

YouTube video of fxIP's IRC client, connecting to irc.libera.chat
YouTube video of fxTerm, a simple RS232 terminal application