ZX Spectrum

Emulators:
Fuse [Mac OS, Linux, Windows];
Unreal Speccy Portable [Mac OS, Linux, Windows, Android, Dingoo, Symbian S60].
ZXSP [Mac OS]
EightyOne [Windows];
FBZX [GNU/Linux];
Xpeccy [GNU/Linux];
JSpeccy [Java].

Graphics:
RetroX [Windows].

AY/YM players:
AY-Emul [Windows];
ZXTune [Linux, Windows, Dingux].

Useful resources:
World Of Spectrum [Global software database];
Virtual TR-DOS [The biggest collection of TR-DOS compatible software].