See also File Library.
Basecamp has a version of kanban they call "Card Tables". It adds a triage row at the top, adds a hidden subcolumn thing on the right for "not now" (for things that don't survive triage) and "done", and has an "on hold" feature that can be toggled for each column so things that can't move forward from a column for whatever reason can live at the bottom.
Client Portal provides a quite clean way to conduct communication with clients around documents and other shared-visibility information exchanges. It doesn't solve internal information management.
Trello also has an API of course, as well as webhooks that I'm sure get fired when cards are moved, something that Basecamp currently lacks for its card tables.