How secure are wireless networks?

If I'm on an open wireless network, can a 3rd party capture packets and see all the internet traffic to and from my computer? What if WPA encryption is used, but someone has the key already? If I use HTTPS, can someone with direct access to the router decrypt the traffic and view it somehow?

If I'm on an open wireless network, can a 3rd party capture packets and see all the internet traffic to and from my computer?

Yes

What if WPA encryption is used, but someone has the key already?

Same as if the network was un-encrypted

If I use HTTPS, can someone with direct access to the router decrypt the traffic and view it somehow?

Unlikely. They would need to have both the Private Key and the Public Key for your HTTPS host, which is unlikely. The other possibility is what's called a "man in the middle" attack, where they replace the certificates with ones that they know, so that it appears to be encrypted, but the man-in-the-middle does a translation on your HTTPS traffic so that it can read it.

Short answer: If you're on a wifi network, don't do anything you don't want to be made public.

If someone is on the same wireless network as you or you are on an open network they can see your raw traffic. Now if that traffic is encrypted, for example with HTTPS, then they can only see the encrypted traffic. As far as I know there is no easy way to decrypt HTTPS traffic.