Wikipedia notes one of the earliest mentions of the falls are found in the journals of Samuel de Champlain who visited the area in 1604, though certainly the falls would have been known to local Amerinds previously.
It's interesting to note in the woodcut above that the two falls appear to be side-by-side, with a thin island separating them, as today, the right side, the Horse Shoe Falls of the Canadian side has been worn back considerably into its now semi-circular shape.
Today, the falls is noted to be not a particularly high falls at 167 feet, but it is extremely wide with high flow water rates that generate hydroelectric power.
K. Bodner, Wikimedia Commons, Public Domain
Since its discovery, the falls has continued to attract increased attention, so much so that by the mid-1800s, dare devils and stunt artists began attempting to go over the falls with life vests, and in barrels, and walking across on tightropes.
Maria Spelterina Wikimedia Commons Public Domain
This postcard picture, shot by George Curtis in about 1876, is of Signorina Maria Speltorina. She walked a tightrope across the Niagara wearing peach baskets on her feet!
One of the successful stunt men to go over in a barrel was Karel Soucek, a professional stunt man who went over the falls in 1984. He was fined for not having a license, and unfortunately died less than a year later in a stunt accident.
The success rate of such stunts was about 75%, and are now forbidden strictly without licence, the last of which was granted to Nik Wallenda (The Flying Wallendas) who did a tightrope walk June 15, 2012.
Niagara Falls made it into film in 1953. Marilyn Monroe starred opposite Joseph Cotton in Niagara, a film noir.
The film showcases Ms. Monroe's beauty alongside the falls. In Niagara she plays Rose Loomis, married to George, and they are visiting the falls. Rose is young and beautiful while her husband is older, jealous and cranky. This stylish crime film of course doesn't end how you think it will!
The Canadian side has a more active tourism segment, notably with the Skylon Tower, two casinos, Clifton Hill (carnival style street), ferris wheel, whirlpool jet tours, Maid of the Mist boat tour, helicopter rides, to name a few. Below is a shot of the Maid doing her run towards the falls.
One of my favourite things to do is cross Rainbow Bridge into the U.S.A. as the views from the bridge are marvellous. Remember to bring your passport if you're crossing, and the cost is .50 cents.
Niagara is especially enchanting by night as she is lit with multicoloured light shows. And every New Years there is a musical show and count down. The falls is about a 2-hour drive from Toronto, depending on traffic.
Mariomaster 1589 GNU Free Documentation/Wikimedia Commons