zxsp is a simulator for the historic Sinclair ZX home computer families. Main supported platform is Mac OS X. Versions for classic MacOS and old versions of Mac OS X are still available from the zxsp download page.
zxsp supports “.sna” and “.z80″ snapshot files, “.tap”, “.tzx”, “.pzx”, “.80″, “.81″, “.o”, “.p” tape files, excellent sound, precise screen and border effects, joysticks, mouse, a virtual tape recorder and virtual keyboards.
zxsp can load from and save to a built-in virtual tape recorder or a real-world cassette recorder or a real-world Specci. It also can dup real-world cassette tapes to “.tap” files or vice versa.
zxsp supports the black&white models ZX80 and ZX81 and the Jupiter Ace, the ZX Spectrum models with 16K and 48K ram, the +128, +2 and the +2A/+2B; including the Spanish and French versions. If you still own one of the emulated machines and find that the simulation does not match the original, you are welcome to contact me for further improvement. Just keep the original thing ready for tests. 
zxsp comes with a debugger with register edit for the main chips, a video beam position indicator, single stepper and memory disassembly. The machine can be throttled down to 1 Hz or overdrived to 20 MHz cpu clock. You can directly load Z80 assembler sources into zxsp.
What’s new:
Version 0.7.2:
• Updated: Included Paul Farrow’s newest emulator roms for ZX80 and ZX81
• New: Jupiter Ace emulation using Paul Farrow’s emulator rom.
• New: Bi-Pak ZON X-81 and ZON X sound modules for the ZX81 and ZX Spectrum
• New: stereo output
• New: volume slider in “Options” menu. (Requires OS X 10.5)
• New: record and replay audio files. (Requires OS X 10.4)
• New: (try to) load tape files from “Open…” menu item
• New: instant load from and save to tape files in the “Options” menu
• Updated: load pzx files v1.0
• Changed: behaviour of “record” button in the tape recorder
• New: context menus in tape recorder and keyboard panel
• Changed: moved “Check Update” to application menu
• Changed: “auto start/stop tape” combined for load and save (with the new “record” button behaviour)
• Fixed: misc. bugs
