Hah! Tons of 'em. I like custom stuff more than printed materials.

For example, I played a cantrip obsessed mage once who invented Double the Fun. It let him cast two cantrips at once. The best part was cantrips were infinite in this game, so I just prepared a nice variety (using level 1 slots to do so) and abused Double the Fun to no end. His favorite thing was to Double the Mage Hands and do a slow clap.

Once (on another character), I came up with a cantrip that let me speak with creatures that were dying (negative HP) so I could ask if I could use their bodies in my undead experiments. I was a polite necromancer.

Another character of mine once developed a spell that let me turn parts of my body into elemental substances, so I could gain advantages. For example, if we needed to cross some lava, I'd cast it on my feet and turn them into elemental fire. Then, the lava didn't hurt me!