I've found gwenview to be awesome. It may not have all the tagging stuff that you want (yet - I think they are working on it) - but it's the best one I've seen for dealing with large numbers of photos. You can also easily integrate tools like krename into gwenview - which is great for me, since I prefer to rename my photos, rather than tag them. It also lets you easily edit the metadata of the photos, which can also be used for tagging.