What kind of project collaboration tools do you use?

I'm using Jira.

The more I use it, the more I feel like I'm the master of functions.

Although it is a Jira company's advertising article, Five reasons why Redmine users should write Jira (English)

We also use Redmine, but we use Jira.

It's not crowded, so I'm using it for $10 as a starter package. I bought it with Confluence and enjoy the joy of documentation.