Ways to play Canasta online include PlayOK and Club Pogo. On these websites, you can find games involving other people or play against the computer. Some websites offer free games, but others require you to pay for a monthly subscription.
PlayOK has a free sign-up, or you can play Canasta online as a guest. Once you log in, you can create a new table or join a game that needs players. Club Pogo offers a one-week free trial and costs $6.99 per month thereafter, as of July 2015. Once you sign up, you can play Canasta against other Club Pogo users or against the computer at any time.