I'm not sure if you're aware that Apple's Remote Desktop server is free. The client software is expensive.
Anyway, for the server, the best (only?) alternatives are Apple's built in Remote Desktop server and OSXVnc, now known as Vine Server. The built in Remote Desktop works fine. I've used both. But Vine Server has more options, including being able to specify only ssh connections are allowed, and also you can specify the display number, which is always :0 with the built in server.
As for viewers, mkrishnan is right about CotV. It connects to both Apple's Remote Desktop server and Vine Server. You can also use any other VNC viewer (e.g. TightVnc or RealVnc) from any other OS to access Vine Server, but you can only connect to Apple's Remote Desktop server with CotV or TightVnc because only those two recognize the version number returned.