Here is my own list of airports and other public places nationally that have free WiFi&trade. These are all hot spots that I have confirmed myself. I will add to this list as new information comes to me (so you may want to bookmark this entry). At the end, I will add links to other people's lists.
Airports WITH free WiFi: SAN, LAS, PHX, CLT, PDX, MCO (Orlando), AVL (Asheville), ROC (Rochester), PHL (weekends only, via AT&T), FLL, SJC, OAJ (Jacksonville, NC), ATH (Athens, Greece).
Airports with "free" advertiser supported WiFi: DEN. (Advertising-supported WiFi is technically free but may be so infected by advertising that it may be unusable for many websites.)
Airports WITHOUT free WiFi: BOS, DCA, ORD, MSP, DEN, SFO, LAX, EYW, BHB, HNL, KOA, LIH, OGG, STT, SJU, PHL, LGA, MIA ($8), SFO ($5). (Most of these airports offer paying wifi, usually for about $8/day.)
Generally speaking, small regional airports (OAJ, AVL, ROC, etc.) are more likely to have free WiFi than the big behemoths, since they see it as a way to promote the airport to business travelers.
I have found NO free WiFi at any airport in Hawaii. The only free WiFi I have found in Europe was at the Athens airport.
Most chain motels have free WiFi for guests, and you might be able to steal some from their parking lot.
Free WiFi at Interstate Highway Rest Areas: All rest areas in Iowa, many in Texas, I-95 Welcome Center in Georgia (at FL border).
MOST public libraries have free WiFi (but a remarkable number still don't). Sometimes, no password is required, but when it is, visitors can usually get a temporary password for free or for a nominal charge. WiFi is rarely turned off when the library closes, so you can often use it in the parking lot after hours.
Most suburban residential neighborhoods are remarkably leaky for WiFi. If you drive around any neighborhood for a while, you'll eventually find an unrestricted signal, and a neighbor will kindly lend you theirs. (Of course, you want to avoid being seen doing it.) Likewise, you can also drive into many office and industrial parks and find leaky WiFi. In Europe, however, free wiFi (of any kind) is extremely rare.
Almost all colleges and universities have WiFi available on at least part of campus, but here's the rub: At bigger universities it is more likely to be password protected, while at smaller community colleges there is a better chance it will be open to all.
(I'm not going to try to keep track of local free WiFi spots, because there are too many.)
Here is someone else's National WiFi list along with their list of Airports With Free WiFi.
Here is an Airport WiFi list from Weather.com (although it isn't always clear whether the WiFi is free).