JustAPIs is described as 'Build APIs Rapidly'. There are seven alternatives to JustAPIs, not only websites but also apps for Linux, Mac and Windows. The best alternative is Tyk, which is both free and Open Source. Other great sites and apps similar to JustAPIs are Fusio (Free, Open Source), APIbond (Free), Tyk Cloud (Free) and 3Scale (Paid).
Fusio is an open source API management platform which helps to build and manage REST APIs. Fusio provides all tools to quickly build an API from different data sources yet it is possible to create complete customized responses.
APIBond is a service that acts as a proxy where you can configure an existing API. One problem with the existing APIs in the market is the lack of customization. Eg. Twitter API calls returns the same response to everyone.
Tyk Cloud is a full-featured API management platform based around the open source Tyk API gateway, that means you get all the features of the open source version without any of the integration and implementation headaches.
3scale provides an out of the box cloud API management platform and infrastructure for developers, and companies to securely open, control, manage, operate and monetize their API to third parties (e.g. developers, business partners, etc.).