Years ago, I made the calendar panel for Opera. I don't really use it anymore myself, but it could still be useful.
I've used Opera as my primary browser since version 7 beta 3 and have known Opera since version 5. It was only when Opera 6 had already been replaced by the Opera 7 betas that I realized that my Javascripts, which I often found a bit problematic to make them work in IE, Opera and Moz, were actually not working in Opera because I had developed them using IE's bugs. I discovered that what I wrote was essentially Opera-compatible and that it didn't work in Opera because I kept having to change things to make them work in IE.