Elyse - The free version allows up to 30 tags.
Quip: Team Documents for Slack - Create, share and get notified about Quip docs in Slack
DEVONthink - DEVONtechnologies develops DEVONthink, DEVONagent, and other Mac and iOS apps for document and information management and web research.
Trainual + Slack Integration - Capture knowledge or access training right inside Slack
Leap - With Leap you find things based on your natural memory of that file.
Papermind - Papermind Slack App is a collaborative article-editing and document management platform for Slack