Your toaster will always be best at cooking bread-based products; there's no getting away from this fact. This doesn't mean you're limited to sliced white, though; everything from bagels to pitta bread can be warmed and heated by your toaster to perfection.
Things that aren't bread:
Many pastry products can be warmed through using a toaster; the ones your toaster can accept are only limited by the size of its slots.
For example, Pain Au Chocolate can be gently reheated in many toasters on lower temperature settings.
Other products are designed purely for use in a toaster, such as "Pop Tarts" and certain "pizza pocket" style snacks.
There are a variety of toaster bags designed to enhance the range of foods you can cook in a toaster. These are heat-resistant pouches are perfect for cooking snacks that may be too greasy to place in the toaster normally - for example, a toasted cheese sandwich.
The bags can even cook food you'd not expect to use a toaster for. For example, "Toastabags" give directions for cooking scrambled eggs in their pouches and state they can be used for warming foods such as pizza slices or chicken nuggets.
Some toaster bags are reusable a number of times; the packaging will tell you more.
Another key benefit is for people with allergies to wheat or other types of flour. By placing a slice of bread in a pouch, you can protect it from coming into contact with toaster surfaces that may be "contaminated" with traces of foods you're allergic to.