For open networks, gathering cleartext data is as easy as sniffing the traffic in the air. This includes usernames and passwords as well as web pages, documents, and other data sent or obtained via http, ftp, telnet, etc. So, I'll just cut-and-paste: On any Wi-Fi network - encrypted or not, given today's Wi-Fi encryption protocols - any sufficiently skilled and equipped user of the network (and especially the network administrator) could easily access any data you transmit or receive via cleartext protocols. That said, the answer I've given there applies fairly well here.
There's a question very similar to this already, but not quite an exact duplicate. Of course, this is illegal, so make sure you're only doing it to test a network's security, or for your own educational purposes. Wireshark is a wifi packet sniffer, which is an essential step in actually breaking into someone's wireless system.
