This should become a sticky thread. So many clever ideas.
On the reaching one, I use a slice. Most things on the top shelves are in boxes. It slides under nicely, and then I can pull the box (think tea, etc.) forward, ensuring it doesn't land on my head as it drops.
I use an Ikea expanding leg (the table now has three expanding and one not, so I will have to grab the leg back if I want a higher table). The leg expands to fit in the bottom of the non-sliding French door and stops the sliding door from opening. You are pushing against the leg. You can buy security bars to do this, but I like "free".
I've added a picture of this one; I can't describe it. These are tea towel holders. They make excellent zip jacks if you use 'zip on a roll' and need to add the zip pulls.