Neil is described as ', a clone of Buzz, is a modular tracker music synthesizer' and is a Music Production App in the Audio & Music category. There are four alternatives to Neil for Windows, Linux and BSD. The best alternative is Buzz, which is free. Other great apps like Neil are Psycle (Free, Open Source), Buzztrax (Free, Open Source) and Buzé (Free, Open Source).
Buzz is the first ever "easy to use" free modular software based synthesizer. What this means is that the entire system is based on objects, which may be routed in a modular fashion, giving you the freedom to be as creative as you want.