Data Pad

In many science-fiction works, even those pre-dating the iPad by decades, the most common form of portable computer is a flat, rectangular slab held in one hand and operated with the other. Usually called a Data Pad or some similar name.

Comic Books

 * The omnicoms of the Legion of Super-Heroes comics are generally depicted as something midway between a smartphone and a tablet computer in size, with a keypad and screen. The ones the Legion uses have quite a few additional functions built in, such as environmental scanning capabilities.

Film

 * Star Wars, though they're mostly featured in the Expanded Universe.
 * Twenty Minutes Into the Future, everyone in the Wim Wenders' film trilogy Until the End of the World seems to carry around a wallet-sized personal computer.
 * In 2001: A Space Odyssey Dave uses one to watch BBC.

Live Action Television

 * Star Trek has PADDs.
 * Star Trek: The Original Series featured the PADD's 23rd Century predecessor, a device usually referred to as the "electronic clipboard," usually given to Kirk by a Yeoman to sign with some sort of stylus, then handed back to the Yeoman. The PADD wouldn't be formally introduced until the 24th Century.
 * Averted pretty hard in Battlestar Galactica, which used laptops until a tablet computer (of a size and shape similar to an early 21st Century model) appeared in the 4th Season aboard the Demetrius. Usually the crew just passes around information on pieces of paper.
 * Caprica replaces these with digital computer paper. In the pilot, Zoe even uses a piece of digital paper to send a text message.
 * "Globals" in Earth: Final Conflict were exactly like Real Life smartphones... years before smart phones first appeared on the market. They were handheld digital computers combined with cutting edge cell phone technology.
 * The titular device in The Hitchhiker's Guide to the Galaxy has, depending on exactly which version of the series you consider, more or less resemblance to modern tablets. The original version had many buttons and a screen four inches square, without a touch screen; the version in the movie more closely resembled a tablet.

Video Games

 * Ubiquitous appliances in the Mass Effect universe, though mainly for transfer of data between individuals. Personal computers takes form of wrist-mounted Omnitools.

Web Comics

 * Freefall has ubiquitous data pads that connect to the global comnet. On the rare occasion that people absolutely need something resembling paper they print out a sheet of plastic.
 * The Cyantian Chronicles "plates" are standard Cyantian tech. Used for everything from schoolwork to indexing subspace inventories to playing games.