The easiest tool I know is YuuGuu. Pro: small to download, easy to use, supports all three platforms Win/Mac/Linux. Cons: java based, slow because it is not peer to peer.
A bit harder to setup because at least one of the peer must reconfigure its firewall is Bosco's Screen Share. This is peer to peer and thus faster. Cons: no Linux support, only Win/Mac. I tested Mac version trying to connect to a peer with port 20229 opened and seems to have problem: it hangs after a while, very annoying. This is why I'm using YuuGuu although it's slower.