I don't know if tag synonyms are cross site, but if not, i think it should be considered. After thinking about it a bit more, Cross site tags, can give som problems, especially for 3 letter acronyms (eg, ASP = Aplication Service Provider or (.ASP files (pre ASP.NET))

After reading about tag synonyms, I think that synonyms are the best solution.

I haven't seen tag synonyms before. After quickly searching around, i think synonyms is a god solution.