New answers tagged categorization
I would allow users to add tags with a link rather than just add them from any text they type, this would make them think about new tags and stop the majority of spelling errors (this should be a comment I know but I wanted to add the mock-up). download bmml source – Wireframes created with Balsamiq Mockups
Atop of @Tom's suggestions, if you involve your community you can crowdsource tag cleaning up. This might be a bit difficult to relate to the authors – other people being able to change MY tags?! –, but sites like StackExchanges and some flavours of image boards (ourobrous and clones) run fairly successfully that way. Without necessarily limiting the tag ...
I can understand your concerns regarding messy tagging, however i think you should allow tagging. There are a couple of things you could implement to guide the user in the tagging process. First of all, i think it's sensible to only allow a small amount of tags, say 3. This forces the user to really think about the tags, and prevents tag lists like vampires ...
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