We have a cheap-o version of a therma pen. Love it, works fine. We also love our clunky-looking but very functional, silicone grill gloves that go halfway up the forearm. We’re using both for our barrel-style smoker rather than for a grill, though.
If you count the smoker as a “grill accessory,” then it’s my favorite. 
It’s a Pit Barrel Cooker, and it’s pretty much idiot proof. It runs on charcoal (we like Stubbs), the meat hangs vertically on hooks, and it makes the best chicken and tri-tip I’ve ever had anywhere.