We'll start with one of the most accurate renderings
Here's an iPad concept that Jesus Diaz at Gizmodo created based on various rumors and leaks at the time. It looks a lot like an oversized iPod Touch, but does resemble the modern iPad much more closely than other concepts we've seen.
This is the real one, we can feel it
Not so much. When this photo made its rounds thanks to French blog Nowheerelse.fr in 2009, it had many people, including the folks over at Engadget, convinced that this was indeed the real thing. We can't blame them for being intrigued— Nowhereelse.fr has a solid track record for picking up Apple scoops even today. The above leak was quickly proven inaccurate shortly after it was published.
Here's a really old one...
This ancient iPad mockup dates back to 2004 long before much of the gossip around Apple's tablet even started. Nearly everything about this concept, which was discovered in an old MacRumors forum post by The Next Web, is completely wrong. At least they got the name right.
The MacBook Touch
Based on the old iPad renders we've seen, this appeared to be a very common concept at the time. Many speculators believed Apple's tablet would simply be a touch-screen keyboardless MacBook running OS X.
This one could even play DVDs...
Another old iPad concept imagined a slate with a Mac interface and an optical drive.
The Macview
This concept created by Patrycjusz Brzezinski in early 2010 dreams up an iPad with a primary display and a second touchscreen that slides out to serve as a keyboard.
Here's one with speakers on the bottom
This mockup created in early 2010 looks like someone just ripped the screen off a MacBook Pro. What makes it different, however, is the visible speaker grill sitting just under the bottom bezel.
The Mac Air
This concept from 2008 is fairly close to what the first iPad actually looked like, but it appears to be substantially thicker than Apple's first tablet. Enrico Dal Pezzo, the concept artist, called this design the Mac Air.
A really slim one...
This claimed to be a leaked photo of Apple's tablet back in early 2010. There isn't really much here other than a screen, but judging by the photo it looks pretty slim. There are barely any size bezels.