I reckon Changi airport in Singapore is one of the best place to spend a long layover: they've got great napping/showering facilities, free video games, a brilliant open air smoking courtyard, excellent shops with good prices, and plenty of space to stroll around. For a domestic layover, any of South AFrica's Slo Lounges at CPT or JHB are awesome - great showers, reading material, top food and drink, free massages, internet, the works!