Google--for-Opera is a project mainly written in JavaScript, it's free.
Google+ for Opera
On August 16, 2011, Google is like: "Opps, nevermind." :|
On August 12, 2011, Google finally removed the Google+ server-side browser sniffing for Opera! Because the Google+ server-side browser sniffing is now gone and because the extensions and workarounds are now unnecessary, you can uninstall the extension or remove the user JS and then reset your user agent back to Opera. Right click > Edit Site Preferences... > Network > Browser identification: Identify as Opera > OK.
If you have any questions, please view the wiki first.