Sync Files
In order to keep my documents, photos, movies and music in sync, I use Dropbox. It is a small program that runs in your menu bar and observes all the documents you want to sync. Whenever you change a file on one computer it will be uploaded to Dropbox and downloaded on your other computers within seconds. There is a free plan with two GB or payed plans from 50 to 100 GB. If you register with the link above, we both will get some free extra space.
Sync Address Book and Calendar
There is a free and super leight weight preferences panel for that: fruux. With fruux you can sync your Address Book, iCal and Safari bookmarks. It uses the Mac OS SyncService (as MobileMe does) and works very good for me. fruux is a student project and appreciates any donation.
Sync Firefox Bookmarks, History and Passwords
As I don't use Safari very often, Firefox is my web browser of choice. If you want to sync bookmarks, history, passwords and tabs, there is an add-on for that: Weave is an extension from Mozilla that helps sharing your data on every computer you use. Weave stores everything on a secure Mozilla server in order to sync, but you also can use your own server.
Screen Sharing
Well, it's not really Screen Sharing. It's more about keybord and mouse sharing. Anyway, imagine you are working on your iMac and want to do something on your MacBook. In order to do that, just move your mouse out of the iMacs screen and you're on the MacBooks screen. Exactly that does teleport. Once you're on the MacBooks screen, you can also use the keyboard which is plugged into the iMac. It's like you have a multiple monitor setup. You can even drag files and the pasteboard can be synchronized. Very handy and free :)
Printer and Scanner Sharing
In the Preferences => Sharing you can enable printer sharing so that other computers can use the printer plugged to your (main) Mac. As of Snow Leopard you even can share your scanner over your home network. With that setup you can print and/or scan documents on every Mac you use.
Internet Sharing
Similar to Printer Sharing you can share your internet connection. Plug your internet cable in your (main) Mac and turn Internet Sharing in Preferences => Sharing on. So you have internet on all Macs in your network although not using a wireless router.