First of all, the integrated BitTorrent client lets you download torrent files using the program's interface. There's no need for other apps.
The Speed Dial function replaces the classic home page with another one filled with shortcuts to your favorite pages. Plus, the email client combines POP/IMAP with an RSS reader and a mailing list organizer. Finally, an IRC client allows you to chat in your favorite chat rooms without leaving the browser interface.
Widgets, thumbnail previews of open pages, content blocking, anti-fraud tools, actions using a mouse and browsing sessions are just some of the features that make Opera a must-have app.