[shkurski_]

It's hard to call it "software", but the last one was a script to merge multiple Google Calendars into one, born out of frustration with juggling them manually. It runs on Google Apps Script, reacts on triggers, so once an event is created/modified/removed in one of the calendars, the change is reflected in the main calendar automatically.

[Edit] In the opposite direction as well: if modified in main, it reflects in the original one. AFAIR, it was the feature I missed, since Google only supports (-ed?) one-way mappings.