Continental bucks the for-fee airport/airline trend and offers free Wi-Fi in all of its U.S. Presidents Clubs: Continental is almost certainly taking advantage of the recent FCC clarification that allows the unfettered use of unlicensed spectrum without a landlord's oversight. Delta, American, and United all use T-Mobile's HotSpot service in their club lounges. Chicago is Continental's one exception: it shares a lounge with Northwest.
Eliyahu Ben-Haim wrote in from Israel to note, Vienna Airport's Austrian and Lufthansa (Star Alliance) Lounges have been WiFi for about a year and remain free. SAS has Wi-Fi in all of its international and domestic lounges, but charges for access via TeliaSonera's service.