I use KaVoom! KM between 2 PCs both running Vista Ultimate 64.

Unlike Synergy, it just works; nothing to configure apart from the name of the PC to connect to.

It costs; but has not caused me any trouble; works well with WoW (no insane spinning that Synergy did).

Edit:
It also works before logging in and I can use either PCs keyboard/mouse on either PC (i.e. its peer-to-peer rather than server-client).
Hmmm, looks like KaVoom also got multicast support (see sticky)...