Although Opera is a modern browser that respects web standards, many developers don't have time to test their sites in Opera and prefer to block it. There are a number of Google services that return "incompatible browser" errors or deliver a trimmed-down version in Opera, even though the site would load just fine without the hardcoded browser checks (also known as browser sniffing). Google knows the service doesn't work perfectly in Opera and other browsers, so it does this to prevent user complaints:
User: "Google, my browser crashed when loading Google Calendar."
Google: "What browser do you use?"
User: "Opera 9."
Google: "Sorry. It's an unsupported browser."
Here are some problems:
1. If you go to Google Docs & Spreadsheets, you get this, an error that recommends adding "&browserok=true" to the end of the URL. There's also a funny disclaimer: "Please note that it is a violation of intergalactic law to use this parameter under false pretenses, so don't let us catch you at it. And, it won't work very well -- really."
2. Google Calendar shows a message box that offers two options: Ok, this browser is unsupported or Cancel, go to Google Calendar and hope for the best.
To fix these problems, you may try to mask the browser as Firefox, but think about this paradox:
"This site doesn't work in Opera because Opera has a small market share. To fix this, you can mask as Firefox. Now Opera has an even smaller market share."
You can read an interesting discussion at Opera Forums. Note that the script is constantly updated and other services that don't currently work in Opera (like Google Page Creator) might be added.