ZX Spectrum

Launched in April 1982 by Sinclair Research Ltd, the ZX Spectrum (or 'Speccy') was among the first mainstream home computers in the UK and is widely credited with kickstarting the British IT industry. On the demoscene today it enjoys a following in the UK, central Europe and Russia. The original model came in 16K and 48K versions; the 128K model followed in 1986, also featuring an internal AY sound chip. All models are powered by a Z80A CPU at 3.5MHz, and have a 15-colour 256x192 bitmap display. Read more on Wikipedia (Photo credit: Bill Bertram)

Emulators for this system includes;

Fuse for Mac OS X, Windows, Linux (and most other platforms)
Speccy by Marat Fayzullin for Windows and Linux (latest version: 3.8, 2015-07-23)
XPeccy for Linux and Windows.

Other useful tools for interacting with ZX Spectrum files are;

inTRD - TotalCommander plugin for interacting with TR-Dos (TRD) disk images.
zx-image - PHP-based ZX Spectrum images parsing into PNG/GIF. Supports animation, supports file caching.
zx-files - ZX Spectrum TR-DOS (TRD) disk image parser.