G-Force is described as 'features fast anti-aliased visuals, millions of possible effect combinations, savable and scriptable effects, and unparalleled expandability. G-Force is designed to entertain you on its own, but there are many ways it can be customized and extended'. There are more than 10 alternatives to G-Force for a variety of platforms, including Windows, Android, iPhone, Winamp and Mac. The best alternative is projectM, which is both free and Open Source. Other great apps like G-Force are MilkDrop (Free, Open Source), Magic Music Visuals (Paid), Avee Player (Freemium) and VSXu Ultra Artiste (Free, Open Source).
Overture Music Visualization is a music visualization experience for Windows, capturing audio in real-time from any source and creating effects that react to your music. It currently includes a stable desktop version, as well as a VR demo that supports the Oculus Rift.
DreamTunes is a gorgeous visualizer. Just make a playlist with the built in player, or let DreamTunes automatically beautify music from any source within ear shot. It’s easy, and your music will look great!
R2/Extreme is a plugin (by Gordon Williams) for the WinAmp music player. It produces animated 3D graphics in real-time that twist and turn with the music. Now used in nightclubs, parties, and even (apparently) a canadian stripclub, R2 has become known for its fast, fluid visuals.
NestDrop allows you to perform with high-resolution high-fps visuals which react in realtime to the music and then broadcasts the video via Spout. Since the Milkdrop engine is at the core you can easily bring in your own Milkdrop presets.