What United States state and federal agencies generally participate in aerial weather modification?
The North Dakota State Water Commission runs the North Dakota Cloud Modification Project, which primarily seems to work at reducing hail damage to crops during the thunderstorm season, and has been active (perhaps not continuously) since 1975.

They've got a section on program evaluations; a peer-reviewed article published in 1997 states:


  [...] the expanded 13-year NDCMP data base showed a 45% decrease in losses in the target area (Smith, et al., 1997).


Their website has a ton of information; a company called Weather Modification, Inc. is the contractor for ND State and does a lot of other work as well. Either the state or WMI would probably be able to provide additional data.

It may also be worth noting that Russia does lots of weather modification work as well (perhaps more when it was the Soviet Union). No idea how successful any of it is.