It depends on what you consider to be "keywords". What you can do as an Administrator is use the "Tag Manager" in Alfresco Share (not sure which Alfresco version this was added and whether you really meant to imply you use the ancient Alfresco 3.3 version when creating your post with that label) to get an overview of all existing tags in the system. Tags are typically used like keywords, and I have seen some customer-adapted user interfaces where the tags field has been labelled as such.
You can also use faceted search via SOLR to retrieve the list of (used) keywords with corresponding usage counts. Lastly, each site / document library has a special "tagScopeCache" node which aims to pre-aggregate the tags and usage counts for that specific scope, but those values can potentially become stale / out of synch over time due to update errors in the backgroud.
