Once the bugs have been ironed out, the features are released as stable, and once every few weeks the streams are all refreshed. You can follow all three if you want, but please note that you shouldn't entrust valuable data to anything other than the released stable stream.
So, what's new in Opera 19 Developer? Amongst the others, we have prioritized some of the things you have been asking for.
- Start up mode option - yes, more options! You can pick one of: last session, speed dial, set of pages
- Pinned tabs - a nice beginning of future tabs improvements.
- Rocker gestures - (left->right click, right->left click)
- Extension APIs - more API stuff to play with: bookmarks, commands, omnibox, webNavigation. Separated article with detailed API's explanation will be shipped later.
- Search engine manager - On top of the current shortcuts to search in the addressbar, (enter the one character shortcut plus your shortcut: "b" for Bing, "g" for google) you can now customize your searches, so go ahead and add "d" for duckduckgo, "m" for your own site, "o" for Opera desktop team blog, or anything else!