RPS Asks: What's Your Rainy Day Game?

It's a bank holiday in fair Blighty, which means the RPS team are all defiantly sat on the beach, clutching our bucket and spade for warmth, as the pounding rain fills the moats in the sandcastles we've built. But that doesn't mean our minds aren't turned to gaming. Right now, as the rainwater washes Alec out to sea, we find our thoughts drifting to our studies. What are our favourite rainy day games? What are the ones we go back to, to fill those spare holiday hours when the sun won't come out to play? Shouldn't someone call the coastguard?

So what are the ones for you? Which game can you reliably fall back on, the one you never delete from your HD no matter how much space you need, the one that can always be depended upon to gobble up the long, dreary hours? What is your rainy day game?