RealVNC is produced by a UK company founded by the team that designed and implemented the original VNC software. Besides the free version the company offer also 2 commercial editions packed with extra features plus a KVM-over-IP based on VNC protocol.
This free software, based on the same VNC protocol, brings interesting features such file transfer and an improved video driver for free. From the same site you can download also several interesting add-ons such as: NAT-to-NAT (allowes for connections between UltraVNC viewer and server behind NAT routers without any router modification) and SingleClick (customizable mini UltraVNC server for help desk support).
This is more about P2P than VNC. In fact the application allows you to establish a P2P connection between 2 firewalled computers. On top of this connection the software runs a VNC derivative (TightVNC). For supporting sessions, the software seems like a perfect alternative to expensive commercial solutions.