Ten important lines (plus Air France's non-380 flights) provide what is essentially a standard approach to premium economy: A seat pitch of 37–39 inches—at least two inches wider than normal economy—plus, in most cases, improved meal and beverage service and other preferred services.
These airlines include Classica Plus on Alitalia A330s, 767s, and 777s, premium economy on ANA 777s, World Traveller Plus on most British Airways transatlantic planes, premium economy on Cathay Pacific 777s and A330-300s, evergreen Deluxe on EVA Air 747s and 777s (EVA was the first line to feature premium economy and it continues to offer an excellent version), premium economy on Japan Airlines 777s, premium economy on Qantas A380s and 747s, economy Extra on SAS A330s and A340s, premium economy on all Virgin Atlantic transatlantic planes, and premium economy on Virgin Australia 777s. Trailing badly is Economy Comfort on Icelandair 757s. Although it's hyped and priced as a premium-economy option, it's really ordinary six-across economy with the middle seat blocked and one inch of extra legroom.
Note that all fares—economy and premium economy—are subject to wide variations by seasonality, demand, and markets. However, a general pattern seems to be that the extra cost of premium economy is less across the Atlantic than across the Pacific, although Japan is a lot more expensive than nearby Taiwan and Hong Kong. But if you're interested, always check for the specific routes and dates of interest.