One little known option is Blue Bar on 36, at the Shangri-La hotel, close to Circular Quay. It's on the 36th floor, is pretty smart (but have been in there in smart-casual) and the drink prices are horrendous, but there's nothing stopping you going up for a look. It's free to enter.
(source: Lonely Planet article)
You get a view down on the Opera House, Harbour Bridge, Darling Harbour and a great view of Sydney Harbour itself.
Another free view that people don't seem to go to - Observatory Hill. You get a really different alternative view of the bridge - so your travel photos are a bit different to the standard Opera-house-from-a-ferry shot, and while nowhere near as high as Bar 36, you are raised on the hill, so have a bit of a good view of Circular Quay and part of the CBD.
(source - my own photo)
The Glenmore Hotel in the Rocks area has a rooftop bar, which is free to get up to, and has a spectacular view of the tourist-y Circular Quay area, with great shots of the Opera House and some of the CBD. Highly recommend for a drink or meal.
(source: Timeout online)
I've been thinking of high-up shots, but if you want just views of the CBD, I can recommend the views from the ferries. Ferries taking you to Cremorne Point, McMahon's Point and around to Balmain East and Darling harbour all have spectacular views of the CBD.
Timeout has a list of the six best ferry trips, and it shows you some of the views