projectM is described as 'LGPL'ed reimplementation of Milkdrop under OpenGL. All projectM requires video card with 3D acceleration and your favorite music' and is an app in the Photos & Graphics category. There are more than 25 alternatives to projectM for a variety of platforms, including Windows, Android, Android Tablet, Winamp and Steam. The best alternative is MilkDrop, which is both free and Open Source. Other great apps like projectM are Magic Music Visuals (Paid), Avee Player (Freemium), VSXu Ultra Artiste (Free, Open Source) and Kauna (Free).
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.
BeatDrop Music Visualizer is a stand-alone implementation of the amazing Milkdrop2 Winamp plug-in. It lets you experience the stunning visual 2D effects with your music player of choice. No additional configuration steps needed! Just start BeatDrop and play your music.
G-Force 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.
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.