Friendly, pleasant trattoria with outside seating just off the square at Gesu Nuovo (the southwest corner, I think). Wide range of pizza, pasta, and traditional Neapolitan dishes; fish or seafood "all brace" has a fantastic char-grilled flavour.
We gave it a try because it was nearly full with students (the university is a few streets away) and locals when we passed by, and as usual, that proved the best recommendation!