Our old friend Devel project has been revitalized and revamped for Drupal 8. Come learn from the Devel maintainer(s) about all its new features and APIs. For example:

Webprofiler

Add a powerful footer to all pages of your site. There, admins can review resource utilization, cache effectiveness, database queries, Views, and so much more.

Core Integration

Quickly access devel features via the core Toolbar module
View info from the Container, Routes, Entities, Events, 
Quickly switch to another user for development and troubleshooting purposes
Edit items in the config and state systems
A twig extension for dumping variables, setting xdebug breakpoint, ...
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Dumpers

A pluggable system for dumping variables onto the page while you are writing code. Kint, Symfony var-dumper, Doctrine are all supported and ship with Devel.