Opera software has released Opera 10.5 Pre-Alpha which is lot more stable and contains big new features. It has noticeably improved with better Windows 7 integration throughout which also includes jump lists, glass and tab previews in the task bar.
With a new JavaScript engine, ‘Carakan’, Opera 10.5 is faster than Opera 10.10. This new JavaScript engine brings it in line with JIT engines in Firefox, Chrome and Safari. If the features are not there, speed is no good and performance is comparatively the same as that of Chrome.

Opera has also targeted latest of the standards which includes JSON encoding/decoding. For developers, revamping the JavaScript feature is a feature worth mentioning, as it has been a sore point in Opera for long. It has got the fastest JavaScript speed than any of the browsers out there.
Opera 10.5 also uses an updated rendering engine, Presto 2.5, which introduces a number of features including HTML5 features like persistent storage and support for CSS3 transitions and transforms. New graphics library supported by Opera 10.5 is Vega, which is currently software based and it displays everything that you see on-screen.
Besides visual changes and use of APIs, for Windows 7, Aero Peek and Jump List support is added so that you could easily access your speed dials, tabs etc. from the taskbar. For Mac, it brings in multi-touch gestures, a unified toolbar, native buttons and scrollbars, and other small features.
You can open a new private window or private tab that forgets everything that has happened on them once closed. Dialog boxes are now non-modal and they are displayed as a page overlay. This would allow you to switch windows or tabs while displaying the dialog. Address field and search field have been upgraded in functionality and also in looks. They remember searches, show results in a better layout and also support removing items from history.
On the darker side, it uses high memory and for Mac build no printing is supported.
Overall Opera 10.5 makes Opera a browse, which developers would want to test with and develop for.