My own custom quirky notation. boxes for hunks of data whether arrays, files etc. Sketchy ideas for classes are just the class name with members and methods written underneath. I deal with hardware often, so the data acquisition system sitting in a car undergoing a test is just a cute little drawing of a car. Duh, simple!
There's a pitch-fork looking thing - a cow prod - indicating when one things provokes another into action, or controls its activity. Arrows for flows of data, sometimes passing through hoops indicated regulation, guidance, redirecting of that data.
I do use arrows of the type used in UML to indicate class inheritance. That is the only thing i stole from UML. (Whatever did i use before i encountered UML?) My background is electronics, so my personal notation is probably based on seeing block diagrams of radios and TVs as a kid.
I am lucky in that about 90% of my work is solo, source code entirely under my command, only giving finished or test versions of executables to others. If i were in a big corporation, i'd be getting a lot of weird looks, like a circus freak!