If you never have it open on the two or more machines you are synching it on, and if it can always update those machines between the time you close it on one and open it on another you can use any of the synching systems out there. I did it for a very long time using the free version of foldershare (foldershare.com), and as long as I remembered not to open it on one machine until the file copied say from the laptop, it worked.

Those two conditions though were not always easy to do, leaving it open in the office on a friday for example would mess up everything at home over the weekend.

When my ISP offered Exchange Server, that made life so much easier. It handles all the synching, everything on multiple machines always sees the same thing, etc.