A nice electronic circuit simulator tool.

A comprehensive note on functional analysis.

A really nice article on how to synchronize game state over a network with latency and high packet loss. The protocol described in the last is quite clever:

Instead of detecting packet loss after the fact and resending lost packets, we redundantly include all inputs in each UDP packet until we know for sure the other side has received them

By the way, the website seems to have a lot of worth reading articles on game development, especially networking related ones: https://gafferongames.com/