Seconding Skritter, they're amazing. I'd also like to add that they support Japanese kanji as well, and that their iOS apps are brilliant. The Chinese iOS app is out, and the Japanese one is in beta. I own a Wacom tablet that works great with the website, but more and more I find myself using the mobile app because it feels even more natural than writing with a pen, and I can do it anywhere.
Skritter seems pretty sweet, but has no love for Cantonese; it supports traditional characters (so they're most of the way there), but the tone selection is mandarin-only and they only display pinyin and not some cantonese phoneticization like jyutping.
