Tongs
Doubtless these are the most important tool you will use, so it is important to get a good pair. Be wary of the cheap scissors type, they have a tendency to backfire, sending whatever you're holding flying into a bush. The tongs should not have sharp teeth, as these will pierce the meat and you will lose the juices. Also check that the springiness of the tongs is right.

Fork
Not the best of ideas - Piercing the meat as it cooks - but they can be useful for rapid turning or removing of food. Better to stick with the tongs though really.

Spatula
These are very useful, especially for turning steaks, burgers and other flat meats.

Meat thermometer
Essential if you will be cooking large meat joints, otherwise a waste of time. The best ones have a reading in degrees as well as "rare, medium, well done"

Hinged rack
Very useful, especially for holding fish together, but also for cooking many small items, as it makes turning much faster and allows for more even cooking times.

Barbecue brush
Especially useful for keeping meat moist, by brushing with oil etc.