Are online casino winnings taxable?

That's a great question. It really depends on where you live and what laws you have to follow. For punters who live in the UK, they don't need to pay any tax as betting tax was abolished many years ago. So, no matter how much they win, they don't need to pay tax. Here's an old news article you can read real quick, http://news.bbc.co.uk/sport2/hi/front_page/1207748.stm

The only ones who need to pay tax are the operators and people who are running the gaming sites. But if you are not based in the UK, then it is advised that you check your local jurisdiction. Hope that helps.