In addition to the commercial hotspots I’ve already mentioned, there are a number of places that I found with free access. The Hotcafé locations all provide free access by means of a code you get from the bartender. There is free wireless access in the Pompidou Centre as well, though you have to register for a free account. That same account appears to work at some of the OzoneParis.net locations as well.
Then there are places, like Quigley’s Point in the photo or the Hotel Novotel Paris les Halles, that provide open access to the internet.