I think honey locust is pretty popular for this use. There are numerous other nitrogen fixing plants ranging from ground covers to trees, so you can tailor them to your system. If your thornless honey locust gets thorny post-coppice or pollarding (it's toward the bottom of the write up) it could be a problem. Honey locust thorns are no joke, they are branched and can get close to a foot long and can pop small tires.

Another nitrogen fixing tree that Edible Acres recommends sometimes is river locust, which seems potentially less thorny. Black locust has smaller thorns than honey locust, makes awesome firewood and its wood is so rot resistant it was poised to displace some of the PT lumber market. Eastern redbud fixes some nitrogen, but it seemed to a lesser degree when I was reading up on it a few years ago. Autumn olive and seaberry/sea buckthorn are nitrogen fixing shrubs, and seaberry has a high vitamin c fruit as a bonus. Sepp Holzer really seems to like lupines as nitrogen fixers, especially on sloped sites because they have a good anti-erosion root system. Then there's the whole clover family, alfalfa, several vetches, or any of the annual peas or beans as herbaceous/groundcover/early succession options.