Analog Box 2 (ABox2) is a free and very powerful modular software synthesizer designed and implemented by Andy Turner. You can download it from his Web site at www.andyware.com. Calling ABox2 a modular synth doesn't do justice to all its capabilities. A quick tour of the many jambots (jamming robots) in the AndyWare online gallery reveals an endlessly fascinating sound-design and algorithmic-composition tool (see Web Clip 1).
Building devices with ABox2 is not for the faint of heart, often requiring you to do the math from the ground up. Fortunately, it supplies a substantial library of prebuilt modules — sound generators, complex filters, sequencers, and so on — and with a little practice, you can combine them in your own creations. Downloading and tweaking jambots and other user circuits is much easier, less time-consuming, and lots of fun. ABox2 hosts VST instrument and effects plug-ins, so you can build interesting jambots without having to create your own ABox2 synths to play them.
Although you can build virtual instruments in ABox2, it's better suited to building jambots. ABox2 has no provisions for routing audio or MIDI output to other software running on the same PC, although you can use third-party utilities to do so. There is a way to capture audio generated by ABox2 in a WAV file, but it's very utilitarian. Recording is probably best accomplished by networking your PC to another computer.
Regardless of whether you're interested in building your own ABox2 circuits, downloading ABox2 and some of its jambots is well worth your time. The files are compact, and the “How did they do that?” factor is high.