Handheld maker Palm has been moving away from its legacy Palm OS for some time now, with its latest product being the webOS-based Palm Pre smartphone. Nonetheless, older Palm OS-based handhelds - including the Centro, Treo, Tungsten, and Zire - have worked fine with the Mac, at least until the release of Mac OS X 10.6 Snow Leopard.
In Snow Leopard, the Palm OS support for synchronization via iSync is no longer present, making it impossible to continue syncing your Palm OS-based handheld with Address Book and iCal. However, you need not toss your handheld into the obsolete technology dustbin just yet, since from Mark/Space can provide  in Snow Leopard. (Reportedly, the ancient Palm Desktop 4.2 continues to work in Snow Leopard as well, though its sync components won't give you connectivity with Address Book and iCal.)
The Missing Sync for Palm OS enables users to synchronize contacts, calendars, and tasks with Apple's Address Book and iCal, as well as with Microsoft Entourage. It can also synchronize music, photos, and videos in both directions. Document syncing capabilities are set up such that you can actually view and edit files with compatible handheld applications such as the Mark/Space Notebook, from Bare Bones, and Entourage. The Missing Sync for Palm OS also lets you archive text messages and call logs to the Mac (for Treo and Centro phones only) for searching or for billing purposes.
The Missing Sync for Palm OS costs $39.95 for new users, and owners of any other Mark/Space product can crossgrade for $29.95.