According to a box of Honey Nut Cheerios announcing the Cheerios line of cereals' 60th anniversary, "Cheeri Oats" were the first ready-to-eat oat cereal.
Today, General Mills's Cheerios are the number one ready-to-eat cereal in America (perhaps because they are so good dry, without milk, fruit, or any other fixings).
Because of their nifty miniature donut shape, Cheerios are a popular baby food. Though one of the few breakfast cereals that doesn't get soggy from lengthy exposure to milk, the O's soften up after a little sucking, which is why even toothless infants can eat them. (Update, 13 June 2002: arcanamundi informs me that the donut shape also makes Cheerios baby-safe in that they're pretty much impossible to choke on.)
Honey Nut Cheerios were the first spin-off cereal based on the plain whole grain oats Cheerios, introduced in 1979. Apple Cinnamon Cheerios, Frosted Cheerios, and Team Cheerios (which combines the regular, Apple Cinnamon, and Honey Nut varieties) are more recent developments (I'll add the years these went to market as soon as I find them out.
Contrary to previous writeups, Cheerios have been advertised by cartoons in the past. "Cheeri O'Leary" was the cereal's first mascot, starting in 1942. The characters Kid and Sue also appeared in Cheerios commercials until 1977. One final funfact: the use of a Cheerios O to dot the "i" on the cereal's logo was first used in 1953.
Source: More than ten years after the fact, I'm guessing http://www.cheerios.com/ and other General Mills-type sites, and/or maybe the back of a box of Honey Nut Cheerios since I did love them as a snack back in the day: just handfuls, straight out of the box, om nom nom nom nom. (3 April 2012)