OutSystems Platform is a comprehensive development platform that enables organizations to rapidly build, deploy, manage, and change enterprise-grade mobile and web applications. OutSystems Platform is used by organizations of all sizes to develop applications for the web and mobile devices, and to quickly deliver them to market. OutSystems Platform includes a powerful visual development environment that allows developers to quickly build apps with drag-and-drop components, an integrated backend to manage data, and a cloud-native architecture that enables fast deployment and scalability. It also provides an optional set of modules that extend the platform's capabilities, such as a low-code platform for rapid application development, analytics, and DevOps automation. Additionally, OutSystems Platform provides enterprise-grade security, compliance, and performance monitoring capabilities. With OutSystems Platform, organizations can quickly and easily build, deploy, and manage applications that meet their unique business needs.
Aptana is not a low-code alternative, but requires programming expertise, while citizen developers can use Outsystems.
Like Mendix, the CUBA platform is a high-level framework aimed at enterprise applications. Applications can be deployed in (a) PaaS clouds. The main difference is that Outsystems is a code generator for Java / .Net, while CUBA provides runtime components, only in Java.
CronApp is a very good RAD platform that, despite being less complete than OutSystems, is a very good option considering its price.
Discontinued Aptana is still under development on GitHub but the latest version, 3.6.1, was released in October 2014. It lacks Java 8/9 features, as well as more recent JavaScript and HTML 5 features.