Really depends a lot on what data you need synced. Some things are really easy (file-based documents), some are harder (iLife libraries).
As the others have mentioned, Dropbox is a good starting point. It's great for keeping your files synced (and it's a good emergency backup, as you can get to it anywhere via the web site), so long as they're just plain files, not dependent on an app's database (i.e. don't try putting your iTunes library on Dropbox). It's also best for smaller files, since you do have to pay for added storage, not to mention the bandwidth/speed limitations of syncing everything to the cloud. It is intelligent enough to do a direct LAN sync between the two computers however.
For syncing larger files (say a video collection), rsync should work well for local sync.
For iTunes, rsync works fairly well at keeping the libraries backed up, but it works best if you keep one computer as the "master" library which handles adding all new music, syncing iDevices, etc. and the other just acts a copy. Check out this question for more details. I haven't tried, but I suspect something similar would work with iPhoto libraries.
For syncing preferences, it depends a lot on which apps. Some have built-in Dropbox compatibility, or at least details on their site of how to get them working with Dropbox. Others use MobileMe (and should be able to make use of iCloud once it comes out in a month or two). For a lot of programs, as long as you don't have them active on both machines at the same time, you should be able to get away with aliasing the preference files to a Dropbox folder.
For email, if you're not already, make use of Gmail. Even if you don't like the web-client, use it's IMAP capability with Mail or other apps to keep your mail synced easily between the devices.
For contacts and calendars, you can use CalDAV sync between iCal and Google Calendar, and you can get Address Book syncing with Gmail contacts, but it can get messy. Once iCloud comes out, this should be much simpler.
Hope that helps and gives you some ideas.