As the demonstration of the Yahoo rich text editor shows (see here) you can easily run web applications in the Notes client. In addition to this however Notes adds value in several ways. One benefit of putting web applications in NSFs is that they can be managed like all other NSFs. You get the great deployment functionality from Notes/Domino, you can set access control, etc.
The key thing that Notes adds though is offline. I’ve seen some solutions from other companies in this space to take web applications offline. They mostly extend the web browsers through plugins and provide small relational databases. The advantage of these solutions is that you don’t have to install another standalone rich client application (as you have to install Notes in this prototype). The disadvantage of these alternatives however is that I’ve seen no built-in replication. I think replication in Notes/Domino is just outstanding. You get replication for data in Notes databases completely for free. This even works between different versions on client and server. Have you ever tried to synch data between different versions of relational databases? It is really hard. And you don’t only get replication for free but you get a highly tuned, optimized and secure replication mechanism.
See here for the source of the hello world sample application.
In my sample I use two web servers. One web server is my Domino server running on my local machine on port 8080. The other web server is nhttp.exe that comes with Domino Designer. It listens to port 80. Before Notes is launched from the Aptana IDE I launch the nhttp.exe from the IDE. The addresses and ports are still hardcoded in my sample and would have to be abstracted in a real product.