Yes, both Slack and Assembla offer a Free Trial.
Slack is designed for Freelancers, Large Enterprises, Medium Business and Small Business.
Assembla is designed for Large Enterprises, Medium Business and Small Business.
Pricing for Slack Starts at $2.67/Month whereas for Assembla Starts at $9.16/User/Month.
Some top alternatives to Slack includes Microsoft Teams, Glip, Google Chat, ChatWork, Dialog, Office Chat, imBee, Takle, SPACE Channel and Teamwork Chat.
Some top alternatives to Assembla includes Quickbase , Smartsheet, ClickUp, Zoho Projects, Jira, Wrike, TeamGantt, GitScrum, Teamwork, ProjectManager.com, Backlog, Axosoft, TaskBranch, Quick Base, Project.co and Height.