Thumbs up to the Plasterers, Kings Head (Magdalen St) and Ribs Of Beef, they're all fantastic and varied real ale pubs.
The Plough on St Benedicts is owned by the guy who runs the Grain Brewery, and there's always a decent selection of their own ales available "straight from the horses mouth" as it were.
Whenever my father-in-law comes down from T'up North we always make a beeline for the Fat Cat on Nelson St. His aim is to do every ale from one end of the bar to the other in one session! Needless to say we've never achieved it thus far, but had a laugh trying.
As a CAMRA member (yeah, I know...) I'd also recommend the Norwich Ale Festival held annually at St Andrews Hall in October. Hundreds of real ales and ciders on offer.