eTask syncs with iCal (mac) and Sunbird (PC) with CalDAV
If you want to fully sync your tasks between your mac, PC, and iPhone using the same open standard (CalDAV) that iCal (mac) and Mozilla Sunbird (PC) use, then check out eTask on the app store.
eTask allows you to view, edit, create, delete, and sync your Tasks on several popular Groupware (e-mail) servers. Currently these include: Kerio Mail Server (KMS), DAViCal, Oracle Beehive, Cosmo Chandler, Yahoo, and SOGo (for primary calendar folders only).
If you don’t have a Groupware server you can create a free account on Yahoo (see the web-site FAW page for details). Once setup you can use the web-interface, iCal/mail (mac), Thunderbird/Lightning or Sunbird from Mozilla.org (PC) to view and modify your tasks. Modifications to your tasks are automatically synchronized between all clients so you never have to take extra sync steps through iTunes or other special software.