This is a vital ingredient for good vanilla ice cream, creme patissiere, and vanilla custard, as well as many other fine desserts. With more milk and less vanilla, this is a great drink (also good with brandy, or as a base for hot chocolate). Recently, some milk drinks are being marketed as "vanilla milk", a supposedly new invention. Actually, this is a well-known drink. It just hasn't been marketed before, because it's so easy to make. Of course, the homemade stuff is so much nicer, anyway.
Split open a vanilla pod, all along down the middle, with the point of a sharp knife. Scrape out the tiny black seeds inside, and drop the seeds and scraped-out pod in a heavy-based pan or (better) a tall microwave-suitable dish. Add (whole) milk: 1-2 cups if making an intermediate ingredient, or up to a litre (4-5 cups) if making a drink.
Bring to a simmer over a low fire, or, if possible, in the microwave (using a low power setting, or the milk will boil over and make a right mess). Simmer carefully for one or two minutes, without scalding the milk or letting it boil. Stir once or twice if you don't want a skin to form on top. Remove from the heat or microwave oven, and stir once to distribute the little black seeds.
Use hot or cold, or any way you like.
Variation: vanilla cream. Go through the same process, with single or double cream, or whatever. Whip it for a luxurious topping, or use as is for any creamy dessert.