The ZX Spectrum beeper is a simple audio coil controlled by the Z80 processor.  It is present in all models but more popular with the 48k edition before the introduction of the AY-3-8912 soundchip (more or less identical to Atari ST's YM2149).

Recommended editors:
Beepola (Multi-engine; Windows, known to run in Wine) http://freestuff.grok.co.uk/beepola/
Orfeus Music Assembler (native) http://www.worldofspectrum.org/infoseekid.cgi?id=0008523
Phaser1 (native) ftp://ftp.untergrund.net/users/shiru/zx/phaser1.zip

also available:
Beep Tracker (native) http://alonecoder.nedopc.com/zx/index.html
Creador Musical II (native) http://www.worldofspectrum.org/infoseekid.cgi?id=0017604
Music Box T.E. (native) ftp://ftp.untergrund.net/users/shiru/zx/musboxte.zip
Music Synth TE (native) ftp://ftp.untergrund.net/users/shiru/zx/msynthte.zip
Stocker (Windows) ftp://ftp.untergrund.net/users/shiru/zx/stocker.zip
YU the Music Box (aka The Music Studio) http://www.worldofspectrum.org/infoseekid.cgi?id=0008487
ZX-7 (native) http://www.worldofspectrum.org/infoseekid.cgi?id=0012538