There are three prominent toolkit vendors that I am aware of, please feel free to add to the list if we missed
1. Salient Process - They have Spark UI Toolkit, External Participant Toolkit, Portal Builder Toolkit
2. BP3 - They have Brazos UI, Brazos Portal, BP3 LDAP Toolkit, BP3 Work Delegation utility, BP2 Email Toolkit, BP3 excel Toolkit, BP2 Gorup Query Toolkit
3. Apex Process Consultants - ​They have four products Apex Coach Views (ACV), Apex Repository, Performance Tools, Firebase Integration