I have a weird fondness for the texture, if not the taste of Velveeta (and Kraft American cheese slices). No other cheese has quite the same amount of slip or smoothness and manages to stay that way, undoubtedly because Velveeta contains sodium alginate, an algae derivative that helps it stay so silky-smooth even as it heats up. It also contains a high level of protein-to-fat ratios, which is what makes it a champion melter.
Thanks to three different kinds of salt (sodium citrate, sodium caseinate, and sodium hexametaphosphate) that are commonly available to aspiring molecular gastronomists, it's now possible to make your own cheese sauces from grown-up, richly flavorful cheeses like Gouda, sharp cheddar, and Fontina that have that unbelievable Velveeta-esque texture.
The fun part is that you can add extra ingredients like beer or stock to give your melting cheeses even more oomph in the flavor department. Think about all the variations of queso dip you could make with this stuff!
For folks who are leery of adding "unnatural" things to their foods, you might want to stick with sodium citrate, which is derived from citrus. It acts an emulsifier, which helps the fat, protein, and water in the cheese stay well-mixed rather than separating. (Sodium caseinate, which is found in animal and human milk, is mainly used by food pros.)
Sodium hexametaphosphate has been deemed as food-safe and is commonly used in many different products, including artificial maple syrup, cereal marshmallows, and even iced tea like Snapple and Lipton.
Fortunately, none of these salts are expensive. So if you want to play around with making your own delicious versions of melting cheeses, you can easily buy sodium citrate, sodium caseinate, and sodium hexametaphosphate.
There are lots of good guides out there on how to make melting cheeses, but this one at Chef Steps is thorough and clear. Saveur also has a great guide on how to make a perfect melting cheese slice. You can also get an introduction to the topic from Chow/Modernists Cuisine below (via Lifehacker).
For more interesting foods, check out our weird ingredients section to learn about citrus caviar, the stinky-yet-delicious durian, the wonders of black garlic, and why Lapsang Souchong tea is the ultimate brining/marinade ingredient.