Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.
Hiver - Manage Customer Support, Sales and Projects right from your Gmail inbox. Collaborate with Shared Labels, Shared Contacts, email notes, email reminders and email templates
Nifty - Manage projects, work, and communications in one place.
Concord - Contract Management Software and unlimited Electronic Signatures. Reduce costs, maximize compliance & mitigate risk. Enterprise solutions available.
SmartSuite - SmartSuite is a collaborative work management platform that enables teams to plan, track and manage workflows, projects and everyday tasks.
Papermind - Papermind Slack App is a collaborative article-editing and document management platform for Slack