ZX-State (SZX) format
ZX-State is the preferred snapshot format for Spectaculator and Fuse. It is usually used with the extension .SZX. Fuse unofficially uses the mimetype x-spectrum-szx for the format (although this could potentially change to x-spectrum-zxstate or x-spectrum-zx-state).
Support for the format is implemented in libspectrum (used by Fuse), Zero, ZXDS, SpecEmu and Spin.
Early versions of libspectrum (prior to 1.0.0) contained a bug which caused the A and A' registers to be swapped with F and F', respectively, when loading or saving in the SZX format. Current versions of libspectrum detect "libspectrum: 0.5.0" and earlier versions strings and correct for this.
There is an extension to the format to add a block type for ULAplus support.