On the Mac, iChat. It has built in Screen Sharing, plus the family can use it to chat with me for non technical issues. And for some, Back to my Mac. I create a user account on their machine with my username, and put in my credentials into that account, independent of theirs. Provides me with a seamless VPN and remote access either via GUI or SSH, allowing for a secondary use of offsite backups for them.

I no longer support any Windows machines, so I'll defer to the suggestions already here from Matt and John.