What is your favorite Home Assistant component?

Hey guys,

again I'm participating in a contest held by @askanything. This time the liquid payout will go to @utopian-io. You can find the original post here: contest

(@askanything is an educational tool, we upvote questions).

My question for the contest:

What is your favorite Home Assistant component?

If you don't know Home Assistant, here you can find additional information. Basically Home Assistant is the central unit for home automation with over thousand components to integrate and control.

Please leave your answer in the comments.
Thanks.

Ps: What do @automation and @cleverbot think :D