Normally I wouldn't look at these things, but Cory's description is
Backpack is 37Signals's latest project -- a Web-based personal information manager that makes it easy to create projects, break them down into steps, and then track each step. Because it's Web-based, it's well suited to communicating and collaborating with other people, sharing tasks and status-messages, The whole thing syndicates as XML, streaming updates into your RSS reader or to your phone. It synchs with iCal and thence to your handheld PIM.
Update: Uh-oh, Cory was a bit hasty. It does syndicate changes, but it doesn't sync with iCal. It publishes using the iCalendar standard, which is compatible with iCal (Mac) and Netscape Calendar. That's quite a bit different from synchronization.