To the right of the normal inventory there is a searchable list of all minecraft blocks and items, including those from any other mods you have installed. Simply click a block or item and a full stack will be added to your inventory. You can add items to a favourites list and even create saves of the inventories you use most often.
In the top left hand corner there are a number of useful buttons that allow you to change the time, change your gamemode and even fill your hunger and health. TooManyItems also includes a very useful enchanting feature to create overpowered tools.