The personal digital assistant. If you have never seen one they looked like this ...
First introduced in 1984 by Psion, the Organizer II kinda looked more like a calculator than the one pictured above. At that point in the PDA's existence, it was considered a pocket organizer. It had a hard plastic sliding cover that protected a non qwerty keyboard and essentially combined an electronic diary of sorts and a searchable address database. Flash forward a few years and many modifications and you get a device that could connect to the internet, had wi-fi, a web browser, allowed you to enter data and had a memory card.
Later versions had touch screens, a stylus, a virtual keyboard, Bluetooth and other "wow" features back in the day. One of the biggest manufacturers of PDAs, Palm, had quite a thriving product line with the Palm Pilot being their most popular model. You could do emails, manage your appointment calendar, to-do list and yes there was a calculator too. In 1996 Nokia even launched one with full mobile phone capability.
Sound like something that exists now? Anyone? Bueller?
By the beginning of the 21st century, the once "amazing" PDA looked a lot less amazing next to the gadget wizardry of the smartphone and are basically now obsolete (Palm shut down their operations 2 years ago and they were bought by Hewlett Packard). Devices got smaller, technology got better and the phones became more affordable and more efficient (minus those pesky data plans).
Think about this though. The company that set the stage for the mainstream PDA (kind of the model that would fuel Palm to design theirs) would be the same company that caused its ultimate demise. That's right, Apple. Their product, The Apple Newton (cir. 1993) was a huge flop ... but .. just a few years later they would make up it ten fold.