This being a site about English language, we should be well-equipped to decide about something like this, eh? :)
Should we prefer singular or plural in tags, in cases where both would make sense?
For example:
- pronoun vs. pronouns
- synonym vs. synonyms
- preposition vs. prepositions
As of September 5th, 2010, the usage is mixed (unsurprisingly).
On the more popular SE sites this is a well-known issue.
Edit: Obviously certain tags (e.g. grammar, syntax, writing-style, etymology, vocabulary, literature, etc.) should be in singular in any case, as plural just wouldn't make much sense—this question is not about such tags.