I have just recently been on board with Opera, but wasn't the Extensions already a part of Opera 11.04? At here: *addons.opera.com/addons/extensions/
Anyway, Opera seems best without any extensions too! Seriously man, with so many features like Speed Dial, Reload Every, Private Tab, Mouse Gestures and what not... I don't feel the need of any "external extensions"
(atleast for now)
As far as download manager is concerned, I use FlashGet for its Grab features, and believe me, now Opera is at its best