I'd answer this in a slightly different way, the question was tagging contributions and not contacts, my assumption is that there is a need to hold metadata against a contribution. We have a similar requirement in the UK for knowing if a specific contribution can have gift aid tax claims made against it or if the contribution already has been claimed etc.
So my answer, assuming this is the case, would be that you first need to add a custom field to the contributions to had the tag/indicator you wish to record about the contribution.
You could then follow two paths to mark them, one is using bulk profile update in a similar fashion to the previous answers. The second method would be to bulk export, tag and reimport. The choice depends on the number of contributions etc. There are relatively minor developments you could also do to make the process much easier - for instance using Rules to automatically mark/update the contributions with the relevant metadata.
Hope that makes sense!