I've got a bottle of Champagne that was left over from last year. Is it still good to serve this year?
By all means, assuming it was not stored near a radiator, in direct sunlight or in some other inhospitable environment. In fact, I prefer to keep most Champagnes (and other quality sparkling wines) in my cellar for a year after purchase. That extra time will usually result in a more mellowed and harmonious flavour. I've known Champagne experts who share my view, too. A fresh Champagne can taste somewhat brittle, while a year-old bubbly tends to display more mid-palate depth and roundness. Think of a harsh digital recording versus a warmer-sounding LP record and you sort of get the picture.
That one-year rule applies to so-called non-vintage sparkling wines, which represent the vast majority of bubblies you'll find on shelves. But there is another category of more expensive sparklers that are designed to improve with many more years in the cellar. These are known as "vintage" sparkling wines and can be identified by the presence of a vintage date (the year of harvest) on the label, such as "1996" or "2002" and so on. Generally these are wines produced only in the best growing years and are thus made entirely from a single year's harvest – versus non-vintage bubblies, which typically are blends representing several years' worth of fruit.
Although fine sparkling wines tend to survive well for a year and often much longer, they should be stored like other wines, in a cool, dark and reasonably humid room. Heat and arid conditions will shorten their lifespan. With that in mind, I hope your Champagne has not been sitting in a refrigerator all this time. A fridge is dark and cool, but it's dry. This could mean trouble for the cork. Air may seep into the bottle through the cork's pores after an extended period, accelerating the aging process. Besides that, you may experience difficulty wrestling that dry, unlubricated cork from the bottle (and I'm embarrassed to say that I speak with some experience on this matter).
The Flavour Principle by Lucy Waverman and Beppi Crosariol (HarperCollins) won top prize for best general English cookbook at the 2014 Taste Canada Food Writing Awards.