Jamkazam is described as 'Revolutionizing the way musicians connect, play, learn and earn. Across a city or across a nation'. There are four alternatives to Jamkazam, not only websites but also apps for Mac, Windows and Linux. The best alternative is Jamulus, which is both free and Open Source. Other great sites and apps similar to Jamkazam are Jamstud.io (Free), FarPlay (Free) and Ninjam (Free, Open Source).
The Jamulus software enables musicians to perform real-time jam sessions over the internet. There is one server running the server software which collects the audio data from each client, mixes the audio data and sends the mix back to each client.
Jamstud.io lets musicians play live in real-time over the internet. Each musician installs an app on the computer connected to he's instruments and browses from any browser to create or join a private or public session.
FarPlay transports audio so efficiently that you can play music, conduct interviews, or practice a comedy routine as if you were together in the same room, even though you might be hundreds or even thousands of miles apart.
NINJAM is open source (GPL) software to allow people to make real music together via the Internet. Every participant can hear every other participant. Each user can also tweak their personal mix to his or her liking. NINJAM is cross-platform.