Spekit is a great tool for augmenting Salesforce's limited documentation capabilities, providing an intuitive way to connect any kind of online documentation or training resources into context-specific help in Salesforce, and provide a single unified documentation dictionary. The value for money is fantastic.
The tool itself comes with robust administrative and management features that allow you to define teams, control visibility of Speks, assign owners and experts to knowledge resources, and track user engagement with those resources. This allows use of the tool to scale smoothly in the same way that the Salesforce CRM does.
Spekit's customer success team is amazing and has invested seemingly limitless effort in helping our company use the tool effectively.
Another huge plus is that the tool is browser-based, and so can be extended to use with any cloud platform -- not just Salesforce! -- that has a defined URL domain. So you can write Speks that are visible/available across multiple systems and platforms. Each spek can be a complete knowledge article, with embedded media, or can link to documentation in any web-based wiki or documentation system such as Google Sites or Confluence.
The Salesforce integration allows for easy documentation of a variety of standard and custom objects (objects, fields, custom picklist values, and admin-viewable components such as validation rules, workflow rules and process flows. And defining specific terms/phrases is always an option. Review collected by and hosted on G2.com.