Others here can speak to this better than I can but here's a start:
Charcoal Grills:
- Weber chimney starter
- Rotisserie (not cheap, looking to get one someday)
- Weber slide-aside lid holder (only if you have a OTS/G w/out a lid holder)
Either Charcoal or Gas Grill:
- Thermapen (not cheap, wonderful for all cooking/grilling/BBQ)
- Good tongs
- Cedar planks
- Rib rack