Opera contains hidden features that aren't exposed in its user interface. They’re on internal pages, which you can access by typing Opera: into the address bar, followed by the name of the page.
Exploring the Opera: Pages
Opera doesn’t have an internal page that lists all its internal pages, like Firefox’s about:about and Chrome’s chrome://about pages do. If you want to view a list and explore them yourself, you can install the Opera Internal Pages extension. It adds a toolbar button that lists all the pages.
You can access some of these pages from the standard Opera menu. For example, the opera:about and opera:help pages are the same as the About and Help options in the menu.
Opera:Config
The opera:config page is Opera’s counterpart to Firefox’s famous about:config page. It contains a wide variety of options and tweaks, many of which aren’t available elsewhere in Opera’s user interface.
The page is completely searchable, so you can quickly find the option you’re looking for. Unlike the confusingly named options on Firefox’s about:config page, Opera’s Preferences Editor page contains options written in plain English.
Opera:Plugins
The opera:plugins page displays a list of your installed browser plug-ins. You can click the Disable option to disable a plug-in without uninstalling it entirely.
The “Enable Plug-ins” check box controls whether plug-in support is enabled browser-wide. It’s the same option you’ll find in Opera’s Quick Preferences menu.
Opera:History & Opera:HistorySearch
The opera:history page displays a different view of your history -- it’s not the same as the History option in Opera’s menu.
The opera:historysearch page allows you to search your browsing history. Like the search option built into Opera’s standard History page, it offers full-text search of pages.
Opera:Cache
The opera:cache page allows you to browse Opera’s browser cache, which caches downloaded content to speed up future load times.
Select a specific website to view its cached files or save them to your computer.
Opera:Debug
The opera:debug page allows connections to remote Opera Dragonfly sessions. You can use this feature to debug remote Opera systems over the network.
To connect to another browser from this page, the other Opera user must launch Opera’s Dragonfly developer tool (Opera -> Page -> Developer Tools -> Opera Dragonfly) and enable remote debugging.
Opera:Drives
The opera:drives page lets you browse your local file system from a web-page-style interface within Opera.
Opera:MemDebug, Opera:WebStorage and Opera:WebDatabases
The opera:memdebug page breaks down Opera's memory usage. The opera:webstorage and opera:webdatabases pages list websites using Opera’s Web storage and Web database features.
Opera’s internal Opera: URLs don’t contain fun Easter eggs like Firefox's or experimental features like Chrome's, but there are a treasure trove of hidden options for you to explore -- particularly on the opera:config page.