Back to my Mac is one of the much-touted features that .Mac subscribers gained with Leopard. Basically, it lets you enjoy screen sharing between two appropriately configured Macs. The main drawback for many is the required .Mac subscription.
If that's you, check out ShareTool. This $20US utility lets users access all of the Bonjour services on their home network, like iTunes music sharing, screen sharing and file sharing from anywhere.
It requires routers that support NAT-PMP or UPnP and Mac OS 10.3.9 (that's right, Leopard is not required). The folks at Bains Software claim that everything is properly encrypted, but we haven't tested this. You can try it out for 15 minutes for free.