There are quite a few different accessories that you can buy for your BBQ.  They vary in price, and some will be more useful for you than others.

• Roasting Rack:  A deep wire rack that is designed to hold pieces of roasting meat.  The rack elevates the meat from the plate so that the meat is cooked evenly and browns nicely.
• Rotisserie:  A long metal rod that is battery operated.  You place the rod through the meat and then turn on the rotisserie.  I recommend putting a basin underneath the meat to catch the fat from the meat as it falls
• Disposable baking dish:  These are great if you don't want to wash up, or if you have a large amount of food to cook on your BBQ.  Convenient.  Usually made from thick aluminium foil.
• Side burner:  A lot of BBQs have these now in addition to the plates. A side burner is ideal for cooking a stir fry or anything that you can cook in a wok.
• Warming rack:  These are wire racks that you use to keep food warm.  You can also use them to  slow cook food through indirect heat.  eg. vegetables
• Smoke box:  Made from cast iron, these are boxes with ventilation holes in them.  You will the box with smoking chips to add flavour to your cooking
• Wire cleaning brush:  Used to clean your BBQ plate.  You can use a paint scraper as an alternative
• Firestarters:  If you are using a wood barbecue firestarters are useful to help get the fire going.
• Cover:  Usually made from lined vinyl these will protect your BBQ from the elements