In order to help prevent too much tag proliferation (many distinct tags that mean the same thing) it might be cool to have a button/link next to the add tag input box that will bring up a popup with existing tags and allow users to simply click on a tag to add it to their post (add the text to the input tags box). To help highlight popular tags you could do something similar to here (http://lumberjocks.com/tags/projects) where the tag text size is based off how often the tag has been used.
SixLinks Developers' response:
This should work similarly to how the inbox deals with recipients.
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