Web Applications
NeXT developed the platform where the World Wide Web was born, and they also led the charge in the development of web apps. In 1996, at about the same time as Netscape Navigator 2.0 was released, NeXT released WebObjects. This was one of the very first web application development environments, and only missed being the first by about two months.
WebObjects was written in Objective-C, and allowed you to use the same model objects in native OPENSTEP (the successor to NeXTSTEP) GUI applications and web applications. It was integrated with the Enterprise Object Framework (EOF), which provided an object-relational mapper, allowing you to visually define a mapping between database rows and objects and then automatically create desktop and web applications with this interface.
With EOF you could create objects whose fields were automatically retrieved from and saved to tables in a database. This allowed programmers to completely ignore the details of database programming and just use the same sort of objects that they were familiar with.
If you've used Ruby on Rails, then you're probably familiar with this way of generating database-driven applications. In fact, most modern web toolkits provide something similar. When NeXT did it, however, it was new.