As I mentioned in another post (“Places to See”), Fraktal has free Wi-Fi for those lucky enough to sit in the front of the restaurant.
Update: Their plug-in spots are all gone. You’ll have to rely on your own battery power.
But there are other places as well around town and I will be trying to list as many as possible here; for example, the Eurest Restaurant in the Siemens building at Evropska 33, Prague 7 (on the way out to the airport) also has free Wi-Fi, and is open to the public. Just remember that the entrance is at the back side of the building. The signal is actually quite good and quite fast, unlike the above mentioned sometimes it works sometimes it doesn’t Fraktal service.