Utilities to help you develop using OpenID Connect and OAuth 2.0 are always useful so I've listed out the ones I know about.
Postman - Really useful to for API testing. Requires Chrome.
v2.0 Protocols - OAuth 2.0 Authorization Code Flow - Article that has links to Postman collection to try this out step by step. Azure AD centric.
Inspecting the JWT token - from Auth0.
Outlook OAuth Sandbox - for testing Outlook.Office.com API.
OpenID Connect Playground - from Auth0 - for developers to test and work with OpenID Connect calls.
Demo. instance of IdentityServer - has full stack OIDC / OAuth2 support.
Tying the two together - hooking the playground up with IdentityServer.