Often I'd like to have a little tools / calculators for various things, mostly for non-tech stuff. If I don't find something for that, then sometimes I make it myself!

[WIP] Shuffle a big deck of cards
Some games can have a massive number of playing cards. Shuffling them properly can be challenging, this might help!

Sun heat through a window
Ever wonder how much heat comes inside through a window? Describe your window and latitude and get an approximation!