Here are some national chains that provide free Wi-Fi access in some or all of their stores:
All Apple retail stores. See http://www.apple.com/retail/ for locations.
Panera Bread is aiming to have all its locations offer free Wi-Fi. In the meantime, many Panera stores already are “unwired.” See http://www.panera.com/wifi.aspx for details.
Many Schlotzsky's Deli locations. See http://www.schlotzskys.com/wireless.html for a list.
Here are a few of the national chains that provide Wi-Fi access in some or all of their stores using a paid provider (this means that you have to pay for your access):
Barnes & Noble stores rely on various different providers. The best way to find a Barnes & Noble location that provides Wi-Fi access is to ...