As blogged about last week, Google is sponsoring free wifi for the holidays for AirTran, Delta, and Virgin America flights.
According to MilesQuest, there are also some free codes out there for wifi on American and US Airways. For American the code is HPAA and for US Airways the code is HPUSAIRWAYS. Unfortunately the expiration date of these codes is unknown.
Update: Apparently United has a similar code — HPUNITED.