Symbian wasn't only incredibly successful, it's also incredibly old. Many people think Symbian originated at Psion as EPOC, but that's not entirely true. Technically speaking, Symbian is EPOC32, which is a different operating system than EPOC. EPOC was written in 8086 assembler and C, and powered many of Psion's PDAs, including the Series 3a pictured below. This thing is a joy to use, actually, even though it's from 1993 and uses a 8086. The built-in spell checker is pretty amazing, and quite fast.
EPOC32 is EPOC's (renamed to EPOC16 or SIBO to distinguish it from EPOC32) successor, and was written in C++. It was a completely new operating system, and introduced features to Psion's PDAs such as pre-emptive multitasking and memory protection, and left the 8086 behind in favour of ARM processors. It was first released in 1997.
Psion would later spin off its software development team as Psion Software, which renamed the operating system to Symbian in 1998. Several manufacturers went on to license and use Symbian, the most prominent of which were probably Sony Ericsson, Motorola, and Nokia. Symbian continued in several forms with different user interfaces (EPOC32 was designed specifically to allow for this), and the last released version is Symbian 3 - which was updated a few times with feature packs by Nokia, the latest of which is Nokia Belle feature pack 2.
And now, Nokia has confirmed the end of Symbian for the company, effectively signalling the end of Symbian as a smartphone operating system. "During our transition to Windows Phone through 2012, we continued to ship devices based on Symbian," Nokia states, "The Nokia 808 PureView, a device which showcases our imaging capabilities and which came to market in mid-2012, was the last Symbian device from Nokia."
And that's that. Nokia closed the source code for Symbian recently, but it has been available as open source for a while - the last code dump is still publicly available under the EPL license.
The end of an era. Goodbye Symbian. I will never forget my Nokia E71 - terrible software hidden by ridiculously classy and solid hardware.