The Apple iPad tablet isn't even two years old, but it owns almost three-fourths of the global market. HP's TouchPad couldn't take it down. That product lasted just two months before a $99 fire sale brought buyers rushing in for the discontinued product.

But now it appears there are two serious threats to the iPad's runaway success: new tablets forthcoming from Amazon and Barnes & Noble.

Not only might the new tablets, priced well below current entry-level iPads at $499, rival Apple by unleashing a new competitive spirit in the fast-growing, youth and young-adult markets, but they are also likely to go head-to-head with each other, clamoring for market share to show they belong in the tablet space.

The first up will likely be Amazon, expected to unveil its first tablet early next week. Amazon's PR firm sent an invitation to journalists late Friday afternoon announcing a press conference in New York City on Sept. 28 when the company is expected to reveal the Kindle tablet.

While the tablet hasn't been revealed publicly yet, it is apparently real -- it has been seen already by one tech journalist already.

TechCruch's MG Siegler wrote earlier this month that he had seen and used Amazon's new tablet -- further suggesting that's what Amazon will unveil at the Sept. 28 meeting. Siegler reported after using Amazon's tablet that the product is going to be a big deal.

So big, in fact, that it's expected to be an immediate competitor to Apple's industry-leading iPad. Amazon has a chance to compete with Apple by selling its tablet at a lower price, offering deals on content, and tying the product in with its Web site, one of the world's most popular.

One study said this summer than one in five Internet users visited Amazon's site in the month of June, for instance.

While Amazon hasn't said when its first tablet will be launched, the company recently redesigned its Web site in a more tablet-friendly manner -- further suggesting an October launch. Amazon's new Web site is less cluttered, with fewer buttons, more white space and a bigger search box. It's cleaner and lighter -- custom-designed for tablet users.

Amazon is expected to unveil an Android-based tablet with some personalized system touches featuring a 7-inch, back-lit screen.

Reports have indicated Amazon will price the new tablet below $300 -- multiple reports suggest $250 -- to compete against Apple's industry-leading iPad and perhaps one or two new tablets from Barnes & Noble, while also providing an annual content subscription model that will let readers have free access to older books and streaming movies and TV shows.

Amazon's tablet library content will reportedly be available to Amazon's Prime members -- who currently pay $79 a year for unlimited two-day shipping for products on Amazon.com and also for access to streaming digital movies and television shows on the site.

Amazon hasn't announced this yet, but the information fits with how Amazon has approached its Prime subscriptions and previous launches of its popular Kindle e-reader devices.

Amazon is expected to sell its new tablet at a hardware loss initially, but it is anticipated to integrate content from Amazon Prime into its tablets to get more members and also to give consumers more value. Currently, Amazon Prime membership doesn't include books, but The Wall Street Journal reported recently that Amazon will likely expand the program to include books for its tablet owners.

Another news journal, eWeek, has predicted that Amazon will also try to undercut Apple with the prices of magazine and newspaper subscriptions for tablet owners by possibly offering publishers lower terms to get the price down.

But Amazon isn't likely to tackle Apple's iPad alone. Competitor Barnes & Noble, America's largest bookstore chain, which also has ereaders and hybrid tablets, called the Nook and the Nook Color, respectively, will release two new Nook Color tablets, according to a report.

The Digital Reader said Barnes & Noble will launch a $349 tablet that might be named Acclaim and another lower-priced model similar to the current Nook Color named Encore.

The report gave no specifications other than the names and the expected prices.

The report, written by Nate Hoffelder, said: According to my source, Barnes & Noble held a planning meeting last week. One of the topics discussed in the meeting was B&N's marketing plans for their ebook readers and how they would be promoted next quarter.

The report said Barnes and Noble's NookColor will be getting a brother tablet. It also said Barnes & Noble is planning to release three ebook readers in the next quarter -- most likely in October. It said B&N will keep the NookTouch at $139, and they will have the two NookColors at higher price points. The Encore and Acclaim tablets will be priced at  $249 and $349, respectively.