What items can you smartcast with?
It's possible to smartcast all items with an active ability (including wards).
As Andrew already explained, you can go into the "Kebybinding," go to the category "Item Inventory," and from there you can set "Smart Cast Items 1-6" to your preferred key (I would suggest taking the numbers 1-6).
List of Items that currently have an active targeting ability (excluding consumables):
- Bilgewater Cutlass
- Hextech Gunblade
- Deathfire Grasp
- Wriggle's Lantern
- Executioner's Calling
- Hextech Sweeper (Dominion only)
Note: All items with an active ability that don't require you to choose a target (e.g. Runduin's Omen) are used by simply pressing the corresponding button to the item slot (no matter if you use Item smart casts or not).