This new syncing app from Mark/Space works with many of the same fundamental apps, such as Address Book, iCal, Mail and Safari, but it employs some unique features. For example, you can chose specific groups in Address Book (instead of simply 'everyone'), and you can then share this information with other users on your home network. Want to let mom, dad and the kids all collaborate and add iCal events for Macworld 2008? SyncTogether has you covered, though it might be a little clunky to work with: local syncs sound easy, but the system requirements state that you must know a machine's physical IP if you're going to sync remotely while you're away from that machine.
For now, SyncTogether is planned to drop Q1 of 2007, and until then, there is least one major question that I haven't been able to answer from Mark/Space's product page: Will it be open to 3rd parties like Apple's Sync Services? As far as pricing is concerned, users who are unhappy with .Mac's yearly subscription model will probably rejoice that SyncTogether's price is already set at a flat $49.95, but those are all the details we have for now.
[Update: Our own Mat Lu clued me in (via email) on the fact that SyncTogether is very likely the product of Mark/Space's acquisition of MySync. MySync, for those who aren't familiar, *did* support any extra 3rd party apps that plugged into Apple's Sync Services, so in theory, apps like Yojimbo and Transmit should already be along for the ride, with no extra work on Mark/Space's part necessary. However, Mat also pointed out, and I agree, that it's kind of strange that SyncTogether's page makes no mention of this extensibility.]