I was looking for a good voltage and current reference instrument recently and I came across this site:
http://www.voltagestandard.com/ which is connected to the articles. They have a small unit named DMMCheck Plus which caught my eye since it also has AC voltage and AC current capability which has always been the problem with my DMMs. I've never had any problems even with some of my 20 year old DMMs on the DC ranges, but the AC ranges have been off a lot (even for instruments that had proper RMS circuitry). I never bought this unit as I had other needs for my instrument but it looks really nice to me.