Google Apps: There's an undo function in almost every Google productivity tool, and even an "undo" for hastily sent emails. But Google Calendar? Google says there's no way to get back events you accidentally trashed. Free web tool Spanning Undelete disagrees.
Once you add this app to your Google Apps account and authenticate it with Calendar, it keeps an eye on Calendar's queue of recently deleted items and backs them up. You can even install it after an "Oh, crud" moment and find the items you just wiped out.
It won't work for those with standard Google/Gmail accounts, but if you're using Google Apps to maintain your own web domain, it can serve as your personal calendar keeper. It's a free tool from the makers of a wider array of Apps backup offerings.