Spectrum-128K and 48K emulator for MS-DOS.
Smallest playable emulator by Vladimir Kladov.

See also Emuz - the first (may be) emulator of Spectrum in the World,
also by Vladimir Kladov: 
http://bonanzas.rinet.ru/zx/emuz.htm

See also EmuzWin, also by Vladimir Kladov:
http://bonanzas.rinet.ru/zx/emuzWin.htm