I have 4 pieces of 1x4x6 steel bar stock that have turned out to be quite handy. They're like a second set of hands for holding something or can be used in lieu of clamps in some situations. Those and my steel center-finding rule are kind of "go to" items that I find I use more than anticipated.
It would be interesting to know what tools other people find to be their "go to" items.
I have a General brand plastic dial caliper. Measures in 1/100" and 1/64". It was cheap, don't remember how cheap, but so cheap I bought a spare. Use it every time I am in the shop many times.
I also have some five pound lead ingots I put four of them on a cable and pulley to counter balance the weight of the DP table. What a wonderful improvement. (DP does not have the crank on the table. Some others are weighting down a tarp on a pile of lumber. They have also been weights on glue ups and many other things.
I made a pair of 6x6x12 angle supports out of 1/2" plywood to support my skil saw when slotting the ends of bed rails for the metal pieces that attach the rails to the bed ends. They are also very handy when doing work where pieces must be supported during fit up check or assembly.
I slept in a water bed for years that was held together with loose pin door hinges, either 3 1/2' OR 4" ones. It was a good way to hold the bed together and it was easy to disassemble when the time came. Just pull the pins and go.