there are 2 worth mentioning
There is a very nice Windows program called CALENDARSCOPE, which in some ways works better than iCAL in its standalone puposes, the interface is handier, better coloring choices.
But unlike iCAL, Calendarscope isn't free, and it lacks the kind of integration that iCAL shares with AddressBook and Mail.
I wish there were a Mac-version of Calendarscope, since iCAL feels very Windows-like regarding (un)stability.
A second choice would be Google's Calendar which apparently is free, and has a version for the web and a standalone offline version called Sunbird. The interface isn't as polished but it might do what you need.
Hope this helps.