The best alternatives to Matterhorn are Freeter, Growpost, and Confleo. If these 3 options don't work for you, we've listed a few more alternatives below.
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Notion— Get over $6,000 worth of Notion + unlimited AI, for free
Growpost Is an Ultimate Project Management Tool For Freelancer. Where You Can Manage All your Projects, Tasks, Clients, or income and Online payments. You can manage Invoices and Client Can online payment Through Invoices.
Confleo is an all-in-one project management and CRM tool that allows you to create and manage multiple projects, assign multiple users within it, create stunning invoices for your clients and even accept payments from them through in-built payment gateways!