The tools I use the most are: Tongs, Spatula for stuck things, A small towel to apply oil, thermometer, a long handled basting brush a brush to clean the grill with spray bottle with water (for flare-ups)
cooking.stackexchange.com/users/2001/Sobachatina
In addition to what others have said, you should have TWO of each meat-handling tool (tongs, mainly).
That way you can use one to handle raw meat, and one to remove cooked meat.
cooking.stackexchange.com/users/2047/Bob
The basic things that you need are: Grill brush to clean the grate Large tongs to get food on and off of the grill With that, oil to oil the grate, fuel, and quite possibly aluminum foil you will be grilling.
A grill basket of sorts can be very useful.
Metal skewers for making kebabs are great if you like that sort of thing and don't want to spend a long amount of time soaking wood skewers
so they don't burn to a crisp.
cooking.stackexchange.com/users/1816/justkt
Tool-wise: spatula, tongs, grill brush (for cleaning) and a basting brush.
If you're using extremely hot fire or high flames there are grill-safe mitts you can buy.
This isn't a tool, but I find buying cedar (or other wood) planks for grilling fish and other meats and veggies on are invaluable.
Similarly, applewood or hickory chips for smoking.
As you get more advanced, there are rotisserie attachments for most grills, but they can get pretty pricey.
I agree with justkt that metal skewers are extremely useful, as well as the grill basket for certain fish and veggies.
cooking.stackexchange.com/users/1236/stephennmcdonald
If you're using charcoal, a spray bottle with H2O can be handy for flare-ups.
Otherwise, I think others have the bases covered.
cooking.stackexchange.com/users/1759/wdypdx22
I have a wide, flat knife that can be used as a spatula; I find it really useful.
Everything else has pretty much been mentioned, but I would suggest a pair of tongs with decent teeth on them.
It's very disheartening when that burger or steak slips out of your grip and into the coals or onto the ground.
cooking.stackexchange.com/users/1879/kecoman
An additional tool that will make grilling much easier is a veggie basket; basically it looks like a saute pan with holes poked throughout it.
Aluminum foil can be your friend, but these little guys are much more flexible.
cooking.stackexchange.com/users/15/mfg
What are some grilling tools that a beginner should have when purchasing a new grill?  Additionally, are there tools that are useful for the more advanced outdoor cook?
What are some grilling tools that everyone should have?
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cooking.stackexchange.com/users/177/AttilaNYC
The most basic tools are tongs, a spatula, a grill brush for cleaning, a basting brush, a thermometer, a water spray bottle for flare ups and a grill basket for veggies. It can be useful to have two of each meat-handling tool, especially tongs. The basic tools are tongs and a spatula. You could also get a basting brush and a grill brush to clean the grill. A grill basket is also a good addition for vegetables.
You will need tongs and a spatula, a grill brush for cleaning, a basting brush and a grill basket for vegetables. You should have two of the meat handling tools.