WebConversions

Universal Constants. . . .http://physics.nist.gov/cuu/Constants/index.html?

Listed Conversions. . . .http://www.monroecc.edu/depts/pstc/parawam.htm

Conversion Calculator...http://ardan.net/conversion.shtml

[Google] also does a good job: just type in something like "proton mass / elementary charge in stone / (fortnight amp)" (without quotes).

CategoryWebsites? see GoodWebsites