Wiki-OS is a pretty interesting project I came across.  They have created an in-browser environment where one can build, compile, execute and share .NET applications.  Sound trippy? It is!  They call it a webtop -- web meets the desktop. Here's how they describe themselves:

WIKI-OS.org is to software what Wikipedia is to knowledge: it is the world's only open-source user-generated web OS where anyone can contribute right away like a wiki. It allows launching rich-client applications directly inside the web browser without installation. It allows viewing and modifying the source code of any application by clicking the Edit button.

WIKI-OS.org includes an online Visual-Studio-like IDE to collaboratively develop C#/XAML/WPF .NET 3.0 applications. For the first time ever, the users of an application can modify the application directly from its user interface (patent-pending technology by Userware-Solutions), and, when someone makes a change to the source code, the application gets instantly updated on everyone's computer.

WIKI-OS.org encourages developers from all over the world to join their forces to build a place where every single connected person in the world can have free, instant, and unlimited access to the best open-source software applications.

Their "OS" includes notepad, a file browser, a XAML Designer, a code editor and some other utilities, as well as a bunch of nifty samples.  Any application that you run can also be "edited" which means seeing the source code for that project.  They also have this notion of friendly URLs, allowing you to link directly to one of their applications or an application you build.  For example, here's a link directly to their version of Notepad: https://www.wiki-os.org/NotepadProject/Notepad. 

The code editor is pretty nice, with color coding, code snippets, find/replace, property panes, events panes and even the ability to reference other people's projects, so that you can componentize applications.  (For example, their IDE reuses the XAML designer application, which they also provide standalone.) 

Speaking of the XAML designer, I was also impressed with that. They support drag/drop XAML creation of most of the basic control (buttons, checkbox, etc.) as well as most of the layout controls (grid, canvas, etc.). Their design surface supports nesting controls in other controls.  And there is also a property pane and events pane for the XAML itself.

They even have a database layer, so that you can store/access information on their server from applications you build.

I spent some time playing with it and was impressed with the performance and general ease of use of the application.  Plus, the spirit of the project is right on, bringing the "view source" mentality to rich-client applications running in the browser.  I have always thought that XBAPs have a wealth of untapped potential; Wiki-os is a great example of what XBAPs can make possible inside the browser.