Free camping for tents and recreational vehicles has been available on Snoqualmie National Forest lands in the top two lots at Alpental Ski Area since the ski area opened.  The ski area clears these lots for access [so be nice as they wish they could make you pay]- but it is part of their lease with the public to keep these lots open for camping and other legal uses.  Follow the Alpental access road past the main lot, past the Maintenance Shop, past the first lift bus stop and up to camp in either of the top two lots.  Bathrooms are located in the third to the top lot and are open 24/7, however water is hard to get from the small taps.  Access to the climbing/snowshoeing trail is at the end of the top lot by the water tower (check avalanche conditions first).

This map is a close up of the upper Alpental lots where camping is free.  The WT signifies the Water Tower, and down by the footbridge (which fell down) is the main lodge and lifts.