Plaxo looks like it might be a good alternative to .Mac sync. The new “3.0” version includes a sync client for Mac OS X that can synchronize OS X address books and iCal calendars from one or Macs with Google, Yahoo, and other web-based addressbooks and calendars.
I’m thinking of dropping my .Mac service when it’s due for renewal in a few months, since the only thing I use it for is syncing and that’s been broken since I started using Leopard. I don’t use my mac.com email since they don’t have any spam filtering and I don’t use my mac.com web page since it lacks PHP or CGI support. With a decent replacement for .Mac sync, I have no further need for .Mac.