Opera is the 4th most popular PC-based browser, with high usage share in Northern and Eastern Europe. They are also the browser for Nintendo Wii and 100 million mobile devices. Opera is well-known for excellent standards support -- their CTO invented CSS, and they are often the first to implement new standards.
I've been working with these guys for awhile, and I recently got the chance to interview Jan and David while they were in Mountain View. We covered a lot of information that will help you avoid proprietary browser behaviors and have your site look as good in Opera as in IE7 or any other modern browser. We also discuss the industry situation a bit.
If you prefer to read text, you can read the transcript here.
Some of the resources mentioned in the interview: