If we have a tumblr page like so. How can we generate a side bar with a list of the tags?
I don't think this is the tag cloud, but maybe that is the only option available. Maybe we need to use the API directly?
A tag cloud is a list of all tags, where the font-size correlates to the frequency (tag name gets bigger if you used it more often than other tags). So you could make use of any Tumblr tag cloud script and overwrite/remove (with CSS) the styling part, to get a "flat" list.
I linked two 3rd-party scripts in this answer: