Opera is a very fast WebBrowser with low-resource requirements. Adware (as of version 5.0); free of charge but with a google relevant ad banner in the toolbar. You can purchase the non-advertising version for ~39USD. The "third" browser on Windows, after InternetExplorer and NetScape/MozillaBrowser. It is currently in version series 9.X. Although it is available on multiple platforms, the Windows and Linux versions are always the newest.
For version 7, Opera Software developed a completely re-written rendering engine called "Presto".
One of Opera's big features is its multiple document interface (MDI). Instead of window pollution on your desktop, all pages open up inside Opera's application frame as views. This interface predated the TabbedBrowsing? features in other browsers.
Opera was also the first browser to introduce mouse gestures to control browsing. It currently offers the most powerful implementation of this, as gestures work not only on the main web page, but also for other parts of the user interface. For example, tree-views can be expanded and collapsed, contents of edit boxes can be selected and copied to clipboard, all with one flick of the wrist.
Another "big" feature of Opera is that it is small and fast. It's about a three megabyte download, and it can run well on a dusty old 486.
Since version 7, Opera has also become incredibly configurable. Keyboard layout, mouse gestures, toolbars and menus are all easily customisable using either an GUI or a simple macro-like language. New toolbar buttons can even be dragged from web pages (e.g. see ).
Other smaller, but not less interesting capabilities include a multi-account web client, integrated download manager, a searchable listing of all links on a page, a searchable history, powerful bookmark manager, research notes, RSS viewer etc.
As very nice feature, you have a Google search box in every browser window. With some tweaking you can configure this search box to work with your favorite search engines (encyclopedias, wikis, dictionaries, etc.) [more info at Opera Search.ini Editor http://starzaki.eu.org/~hclan/operapl/en/]
It also has extensive panel (sidebar) capabilities.
Opera includes : (far from exhaustive list follows)
Though not OpenSource, Opera support OpenStandards?
WikiPedia:Opera_web_browser has more details.
For reviews, see