I use the Notes box in almost all of my games.

If it's a Wiki tourney, I put the link in the box.
For Hangman and variants I put in solutions and possible solutions. If it's Thematic Hangman I will put in there what the Theme is: Books, Actors, MLB, etc...
For all of my card games with bids, I put in the number of tricks I'm bidding, and especially make it a large type if I bid Zero in Spades!

Bid 0! Be careful!!

If I've figured out that someone has the dreaded Q , I put that in the box.
For some reason I always confuse 3-handed Whist with Hearts, so I put in the box:

WHIST Not Hearts

In the various Ludo games, I put my color and position in the box.
In Senet and Fast Chinese Checkers, I put my color in a large font so it's immediately obvious. [a large font is made by placing one or two equal signs [ = ] at the start of the line.
In Rummikub and variants, I will make note of melds waiting to be placed so I don't accidentally get rid of tiles that I want to remember to keep!
In Cribbage, I put the cards I placed in the Crib in the box.
I test out jiggle_letter_phrases in the box. Nobody sees my ~~failures~ that way!
I'll often use it to test links that I want to include.
In games that are using special rules [Like Around the World CrossDowns Race] I put the rules up there.
In my CrossDowns games, I put in reminders of words that I'm setting up or waiting for letters.
In new games for me, I put in reminders of specific rules I need to remember.
and most importantly, if the game is just between me and one of my many friends on the site, I put FUN in the box.
Some people [Rabbitoid for one] use the yellow box for the xiphias variants, to keep track of opponent finances.
RobynHode uses it for Domination writing something like "Move on Africa next" or "Watch Green."
Bill Roupas uses the yellow box in games such as pente or golden pente or reversi to remind him of a possible good next move.
Others use it for Chess.

The possibilities are endless!