The National Park Service will once again allow entrance to all 401 National Parks for free on select dates in 2014. (Glacier National Park pictured.) That's a savings of up to $25 (at Grand Canyon National Park, the most expensive national park). The next free days are February 15 to 17.
Of note, admission to most national parks is usually free; click here for a list of parks which offer free admission on these days only.