Pundits are predicting that Apple will sell 21 million iPads next year, which isn’t bad for a gadget that some people wrote off as a dud when it was first announced.

The iPad’s popularity has opened other manufacturers’ eyes to the appeal of compact tablet computers though, and some information slipped out this week that shows what the competition has in store.

First up is the Samsung Galaxy, due for release at the beginning of November. With a 7” screen, this is smaller than the 10” iPad, but there’s a clear resemblance in the overall design. Despite running the Google Android operating system, there’s some similarity in the way the Galaxy works, too — as this nine-minute video demo shows.

The only catch is the price and the Galaxy, with its built-in 3G modem, costs £599.99 at Amazon — and that’s with a £200 discount. The 3G iPad, on the other hand, costs £529 and the Wi-Fi-only model is £100 less.

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Next is a video that shows a prototype the HP Slate in action. This was first seen at a US trade show at the beginning of the year, when Microsoft unveiled it just before Apple’s iPad launch. The 7” Windows-powered Slate was thought to be a dead duck after HP bought smartphone maker Palm a few months ago, so it’s unclear what this demo video is showing. As the video makes apparent though, the Slate in this particular form poses no threat to the iPad — after all, it has a Ctrl + Alt + Delete button…

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Finally, BlackBerry maker RIM is rumoured to be working on a tablet too, but details are scant at this stage. Some reports reckon this could be announced as early next week and will most likely take the form of a 7” touchscreen device that will use a different operating system to the BlackBerry smartphone, but it’s all speculation at this stage.

There are a handful of other tablet computers in the pipeline from other manufacturers, but we’ve yet to see one that’s as slick as Apple’s effort — maybe 2011 will be the year that all changes.