The following plug-in (WP Calais Archive Tagger) is very similar to, and in fact written by the same author as the Calais Auto Tagger plug-in I mentioned back towards the end of September.
It too analyses posts for you and tries to extract the right terms to use as tags. However, whilst the Auto Tagger works with posts that you are actually editing at the time, the archive tagger (you may have guessed this part already…) trawls through your archives (old, previously published posts) and adds any suggestions it finds. Again, you’ll need to sign up for a Calais API key, but these are free and I don’t appear to have received any spam after signing up with the scheme. Since I first installed this plug-in, updates have been released, so your old tags aren’t wiped out by the new suggestions, and throttling has been put in place to ensure you aren’t banned from Calais’ server by making too many requests at once. Oh, one quick note, WordPress’s dashboard has altered since the plug-in was first released – I don’t know if that is the reason why the area to enter your API key has moved to within the Plug-in section, rather than under comments, but in case you struggle to find the right area, now you know.