If you want open source, and you're okay with manual sync and conflict resolution, I've had good results in the past using Unison[1] to sync multiple systems.
At this point, though, I've personally settled on a system similar to the GP's — a backed-up laptop. In the rare event I need to access a document on the laptop when I've left it at home, there's always ssh, and, in the even more rare case where I need a document and I've left the laptop elsewhere, there's always ssh + rooting around Time Machine folders on my backup server. Finally, for times when I don't want to carry the laptop and know I'll need access to files, GoodReader[2] on iOS syncs over a variety of file server and cloud storage protocols.
