Wifi Liberator lets you retransmit the pay-for-use WiFi at airports and cafes to your friends for free; it's an art-project from Jonah Brucker-Cohen:
Wifi Liberator is an open-source toolkit for a laptop computer that enables its user to "liberate" pay-per-use wireless networks and create a free, open node that anyone can connect to for Internet access. The project is presented as a challenge to existing corporate or "locked" private wireless nodes to encourage the proliferation of free networks and connectivity across the planet. The project was inspired by the ongoing "battle" between providers broadcasting wireless signals in public spaces, in particular: corporate entities, wireless community groups, individual users, and proponents of open networks. Like my Wifi-Hog project, the Wifi-Liberator critically examines the tensions between providers trying to profit from the increasingly minimal costs associated with setting up a public network and casual users who simply want to see the Internet transform into another "public utility" and become as ubiquitous and free as the air we breath. The project targets pay-per-use wireless networks as often found in airports, other public terminals, hotels, global-chain coffee shops, and other public waiting points.