I recently found out about Connectify, which can create a full (as opposed to ad hoc) access point using only a computer. It also allows to share an internet connection from another WiFi network.
Thus, the computer can be "connected" to two WiFis at the same time -- the one it created and the normal one.
How is that achieved, with only one physical WiFi card? This also brings up the question, would it be possible to write a program that would make your computer be able to connect to multiple (normal) WiFis at the same time (without creating any of them, like Connectify does)?