Whimsical's starter plan gives you a limited number of free items per workspace, in up to 2 free workspaces.
They also offer monthly and annual subscriptions which allow unlimited items per workspace. More information can be found here.
Whimsical is a "unified suite of collaboration tools" that allow its users to create documents, flowcharts, wireframes, mind maps, and sticky notes in a visual workspace. The company is headquarted in Denver, Colorado, and has deliberately chosen to bootstrap their business rather than take venture-funding.