Slack can be categorized under Agile Project Management Toolss, which is commonly listed under Agile tools.

Not all similar tools offer the same features. Most include a free plan or trial, so you should be able to give them a try. You can browse some of the alternatives to Slack listed in the directory.

Some tools might also offer discounts or offers, especially for newer members. Most of these offers traditionally come as part of affiliate programs or promotions, but often you can reach out to companies and check whether they can give you special pricing.