Trello

Trello is the type of tool you can open and start using within 5 minutes. It gives you a kanban board or pipeline-type view on which you can create 'cards' and put them into 'lists'. Each card can hold checklists, notes, comments, dates, tags, custom fields, and files.

You can even CC cards when emailing so that all email content gets saved to the card. Along with a simple and intuitive user interface,

Trello offers a number of easy to install integrations such as Slack for notifications, Butler for automation, and Screenful for reporting and dashboarding on analytics. This is one of the most extensible yet easy and intuitive tools I know of.

Great for CRMs, Issue Tickets, Project Management, and anything else that has a process or pipeline that issues move through.