Asian chile sauce
A Thai-style chile sauce, Sriracha is made from sun-ripened chiles, garlic, sugar, salt, and vinegar, giving it a hot-sweet-sour-salty flavor profile. Its ketchup-like consistency makes Sriracha an ideal condiment to add a jolt of heat to sandwiches, soups, and other dishes.
To learn more, read Tony Rosenfeld's The Spice Lover's Ketchup for additional uses for sriracha, including adding it to dips and bloody Marys, and view a slide show of Essential Thai Ingredients.
Substitute another Asian chile sauce or hot sauce.
Huy Fong is the most widely available brand in the U.S., with its distinctive green squeeze top. It's sold in many well-stocked supermarkets (check the Asian foods section), or in Asian groceries.