I'll admit that I'm a sucker for a good kitchen novelty, and it was certainly that affliction that initially drew me to the Boska Holland Toastabags, but it turns out they're both practical and really useful too.
These "toasting bags" are synthetic envelopes that create a near-perfect grilled cheese sandwich using just your regular toaster. You can even add little a ham, or perhaps use a pita with some veggies instead. I was skeptical of these claims, as found on the product's packaging, but they're true.
You simply assemble your sandwich as usual, but sans butter, then slip it into a Toastabag. Insert the whole shebang in your toaster and drop the lever. After one medium-darkness cycle, your sandwich will be hot and the bread toasted — complete with little grill marks.
The secret is the envelopes, of course, which are made from some sort of conductive material that amplifies the heat from the toaster's elements. (Apparently woven fiberglass coasted in PTFE, AKA Teflon.)
The bags work best with a wide-slot toaster, and slim bread, but I've managed to stuff even thicker slices in the bag with just a little effort. Be sure to monitor the "grilling" process, at least at first, as it happens a lot faster than you'll expect. The bag is hot when it emerges; probably too hot for some children.
The manufacturer claims that the bags are free of any mess, and they certainly don't muck up your toaster, but they do such a good job of melting the cheese that some will ooze out inside. No problem, just let the bag cool and turn it inside-out to clean the non-stick surface.
The video at the product's website presents a fair and honest depiction of both the process and results. You get a pack of 3 or more bags in a set, each of which are reusable. I have yet to wear out my first one; they are said to be good for at least 50 cycles.