On July 5, the USDA's Economic Research Service (ERS) released a new set of data on adoption rates of transgenic crops in the United States. The results are stunning: more than 85% of cotton and soybeans planted and almost 75% of corn grown in the U.S. have been engineered to include genes from other organisms, such as the soil bacterium Bacillus thuringiensis, or Bt, which kills certain insect pests; others confer resistance to popular herbicides (like glyphosate, a.k.a. Roundup, to which some weeds have already developed resistance). As biotechnology's proponents like to point out, almost all domesticated plants and animals have been crossbred to promote desirable traits, making them "genetically modified organisms" or "genetically engineered." The term "transgenic" more accurately describes the products of the invasive, cross-species gene splicing that can occur only in the modern lab, not in the field, but the terms "GMOs" and "GE food" are more typically used.
Soybeans and corn are everywhere in the nation's food supply as animal feed or as building blocks for industrial food (both are ingredients in light mayonnaise, for example). Corn is also in your gas tank: in many places in the U.S., corn-based ethanol is blended into gasoline in small quantities (generally less than 10%). And since soybeans are the preferred raw ingredient for biodiesel, transgenic soy biodiesel might be powering the truck that delivered some of the goods you use.
At this time, neither the USDA nor the FDA require labeling of foods containing transgenic ingredients. To avoid genetically engineered foods, it is necessary to either buy only USDA Certified Organic products, or to rely on voluntary labeling by manufacturers, a practice that hasn't been banned yet (I'm sure that lobbyists from Monsanto and other biotech firms are whispering "ban GMO labels" in the ears of lawmakers and regulators). A small percentage of foods — primarily from natural food companies — carry notices that "no GMOs are contained in this product." For example, the two brands of tofu that I buy (House and Sacramento Tofu Company) have small messages stating that "This tofu was made with soybeans that have not been genetically modified."
The ERS website has a lot more information, such as the split between the two major types of genetic modifications (herbicide tolerance and inclusion of Bt genes), a glossary of biotechnology terms and data files with state-by-state data on transgenic crop plantings.