Opera, the browser
http://www.opera.com/
Download the Opera browser and experience how much better a browser can be.
http://www.opera.com/support/
New to Opera or have a question on the browser? This is your starting point.
http://my.opera.com/community/forums/
Didn't find the answer to your question in the previous link? Post it in the forums (Don't forget to search them first).
http://www.opera.com/support/usingopera/operaini/index.dml
An explanation of the 'opera:config' screen for advanced customizing.
http://www.opera.com/support/search/view/454/?keyword=developer%20console&platformID=10
A good introduction to what may make Opera display a page incorrectly.
http://dev.opera.com/
"Dev Opera is a community resource site where developers can share tips, tricks, extensions and more."
http://dev.opera.com/tools/
Two very useful tools for website developers.
http://www.opera.com/press/faq/
Destined for the Press, but an interesting read for anybody.
http://operawatch.com/
"Opera Watch, an Opera browser blog, is run by Daniel Goldman, who works for Opera Software as a Technical Evangelist." If you are new to Opera, be sure to see his "Opera tips" section.
http://my.opera.com/Rijk/blog/2006/07/04/top-150-popular-firefox-extensions-and-opera
Rijk's list compares Opera's built-in functionalities to Firefox's add-ons. Particularly handy for those coming to Opera from Firefox.
http://operawiki.info/WebDevToolbar
Also for web developpers: Use this in conjunction with the Opera Developer Console and DOM Snapshot tools (above)
http://www.pallab.net/2006/04/12/extending-opera-the-ultimate-guide-to-customizing-opera/
Thought it was impossible? Probably not. But as Pallab points out, finding out how may be difficult. Luckily he's brought together a LOT of helpful advanced customization sources here.
http://my.opera.com/Lex1/blog/flashblock-for-opera-9
From A.Ruzanov's blog; if you've just had it up to here with useless flash animations.
The other side of my brain
http://wanderingelectrons.wordpress.com/
My blog for subjects other than Opera / web technologies. Give it a visit to explore the other corners of my noggin.
Wanna make a web site?
http://www.webdevout.net/articles/my-site-doesnt-work-in-x-browser
This (relatively) short article on why a site can work in Internet Explorer and not in other browsers, such as Opera or Firefox, should be prerequisite reading for anyone wanting to make a webpage.
http://www.htmldog.com/
This is a how-to site that I found (and still find) very useful for learning HTML and CSS
http://www.w3schools.com/default.asp
Although not to be confused with the W3C (see below), this how-to site is also quite useful. It provides most notably web-based interactive tools that you can use to grasp basic concepts quickly.
http://www.w3.org/
The W3C is, if you will, the "Governing Body" of website development. Sometimes difficult reading, but this is the source of all standards compliance.
http://validator.w3.org/
If you want your website to display correctly in all browsers, it needs to compliant to the standards established by the W3C. You can use this validator to test your HTML markup.
http://jigsaw.w3.org/css-validator/
Your CSS must also be valid to insure correct cross-platform display. Check it here.
http://www.alvit.de/handbook/
This is a pretty handy useful-links site for those looking for web development specific sites.
http://www.alistapart.com/
As they say it themselves, "'For people who make websites', A List Apart Magazine (...) explores the design, development, and meaning of web content, with a special focus on web standards and best practices." Essential reading.