What are the unused but useful feature you know in D?

[Gary Willoughby]

On Sunday, 25 June 2017 at 23:21:25 UTC, aberba wrote:
> Can you share feature(s) in D people are not talking about 
> which you've found very useful?

Some of the best features are in the standard library. I've 
written about them here:

http://nomad.so/2014/08/hidden-treasure-in-the-d-standard-library/
http://nomad.so/2015/08/more-hidden-treasure-in-the-d-standard-library/