1. Tabbed Browsing and Pop-Up Blocker -They were the first to intoduce these features
2. Integrated Search Engines – Search through Google, eBay, Amazon , Answers.com , BitTorrent , Download.com and many more
3. Strong Security – Protects you from spy-wares , viruses , malicious applications and phising attacks
4. Speed – It claims to be the fastest browser on earth openly! Also Check Here
5. Password Manager – Stores username and password for each page or site and signs in automatically on a click of a button
6. BitTorrent – Only browser on Earth to support BitTorrent downloads
7. Mouse Gestures – This feature allows you for completeing common tasks using mouse movements
8. Customizaton – You can change the browser’s look almost in any way you want by skins and other appearance options
9. Sessions – It automatically remembers the sessions ,i.e the web pages that were opened before closing the browser and the scroll position of the pages
10. Voice – The first browser to prepare for a future of Web sites offering interactive, voice-enabled shopping and booking systems..and you can also command the browser like next link, select text , blank tab and many many more!!
11. Opera Mail – Built-in POP/IMAP E-mail client is a combined e-mail program, news reader, mailing list organizer and RSS/Atom newsfeed reader
12. IRC Client – Communicate with people all over the world using Opera’s IRC chat client. Chat privately or in rooms, or share files
13. Widgets – These are small programs providing special features, which can be directly integrated with the browser…hundreds of them to choose from
14. Additional features – Full support for Gmail , First Web browser to natively support Scalable Vector Graphics (SVG)
and many more features… cause now i am tired of writing more
but you do give this browser a shot you will forget every other browser be it mozilla, IE , Netscape or ….