You have to subscribe (monthly or annually), but the New York Times daily crosswords and other puzzles are all online, including their archive of over 5,000 puzzles. You can play with friends, time yourself against others, ask to reveal a square or word (or just check to see if your entry is correct). There's also a separate app to play on mobile devices.
Great format, great selection. If you're not familiar with them, the Monday one is the easiest, progressing to the most challenging one on Saturday and then the big Sunday puzzle.
They do offer two new puzzles from their archive for free every Monday.