Vortex Tracker II is a Win32 tracker used to make music for the AY-3/YM series of sound chips, used in the ZX Spectrum and Atari ST home computers, among other things. VTII can also synchronize two open modules to emulate dual AY playback, which is possible on the real hardware via TurboSound or TurboSound FM.
VTII has its roots from Pro Tracker 2.x. and 3.x. (no relation to the eponymous tracker on Amiga), and most of the features and backward compatibility come from the latter versions of PT: pure VTII has the same features as 3.6., but it's also possible to save modules under 3.5. or 3.7.
This thing can also open a huge variety of ZX Spectrum AY tracker formats, including the relatively obscure ones. Yerzmyey dispenses some advice on the various formats in this thread. However, in case when you need to save an unfinished songs and keep all the unused instruments/notes/patterns from being cleaned up, there's also a possibility to save these as .TXT files. Which is also useful, for instance, if you want to make a song on Stocker beeper engine.
Nowadays, it's not used exclusively as a ZX Spectrum tracker anymore - with the help of AYemul, it's possible to convert the song to .psg (which is a less CPU consuming but more sizey AY format - although it's possible to squeeze it with PSG Packer by psndcj/Triebkraft & introspec).
An unofficial VTII interface mod has been made by siril/4d, and it's known as Vortex Tracker II Improved. The most obvious and useful fix over here would be showing envelope numbers as actual notes (without having to type the said numbers out manually). Since it's hardly googleable, Tayle is kind enough to give it out right here (although he's also wary enough to recommend you double-check it for malware ^,^).
Tutorial by BotB's own ant1!
Vortex Tracker II fan art