it's free.
www.realvnc.org is the flavour I prefer ( the original version).
But there are variants on it with various compression tweaks and ui's on them.
Only thing to bear in mind; keep the same version on the server as on the client. Sometimes the server can quit if you try talking to it with a newer version. At least with older releases of realvnc it would do this. Doesn't seem to with the latest releases, but why risk it
