I'm sure we all have them.

Here's one to start.

I've started taking a picture of my incident meter just before or just after a set of shots in one lighting situation.

This "meter shot" can give me a great way to remember what I did and a nice reference shot to set my enlarger to.

Mine is a Seconic L-358. It is gray (like a grey card I can peg mid-tones), has a white dome (white point) with a black ring around it (a black point) that can be used to check contrast and exposure.