DRDUL I've been using this for a while on my Mac Pro. This week I'm travelling with my MacBook Pro, on which I forgot to install iCal Reply Checker. After not being able to properly manage iCal's behaviour with two meeting invites, I couldn't stand it any more. I downloaded and installed iCal Reply Checker and all is well with the world. The moral of the story: don't leave home without it!
Why do I find it indispensable? Because far too many people mess up their meeting invites, especially when they reschedule or cancel. Maybe it's because most of them are using Outlook. iCal likes to reply to everything, and as a result I looked like a doofus because I'd be firing out replies left, right and centre. With iCal Reply Checker, I have the option of preventing iCal from replying to botched meeting invites.
(Version 1.2.1)