You've probably already heard, Opera 11 will support extensions. I decided to try out their new extensions API, Opera 11 is still in alpha stage so everything concerning extensions is still buggy and I had to struggle with some bugs.
Two extensions that I converted from Chrome to Opera are NoFollow and Block Yourself from Analytics. I uploaded extensions for review 7 days ago, after about 5-6 days of waiting moderator finally posted a message, unfortunately I had a small issue where adding websites could be confusing to users which meant waiting for few more days after I corrected the issue. The extension NoFollow was successfully released tuday, soon after that extension moderator also approved my other extension Block Yourself from Analytics.
NoFollow
NoFollow extension is almost identical to Chrome extension, the only difference is that you set options in a popup window because of the (hopefully temporary) lack of options configuration for extensions in Opera. Only known issue is that popup window is not closed after you click on Save or Cancel button, this is Opera bug.
Block Yourself from Analytics
Block Yourself from Analytics is different from Chrome extension, the difference is in the way users enter web sites to block analytics javascript. While extension for Chrome uses regex strings, users just enter the domain name in extension for Opera. Reason for this change in Opera extension is because of complaints Chrome users had for using regex to enter web sites. Other difference is same as for NoFollow extension, options are set in popup window.