ideone: online IDE
A few weeks ago, my friend showed me very cool thing: ideone. It is online Integrated Development Environment for many languages. Of course, it is not that big like Visual Studio, but enough for simple programs.
It is useful for me, when I am on Mac, and I want to test some C# code (but I do not have Mono installed). It might be useful for you when you are on your tablet (e.g. Surface or iPad). Moreover you can use it on your Smartphone! I tested it on my iPhone 4:
* Above screenshots were made on my iPhone 4.
What I really like about ideone is its UI and performance. It is designed to test small pieces of code. You can save your code pieces, share them and fork other users' code! Additionally ideone supports many programming languages (even Ada!):
In last year, Lukasz Gasior recommended online IDEs for Web Developers to test HTML/CSS/JS and collaborate. You can check them here: 10 HTML & CSS Online Code Editors for Web Developers. Most of them are not that lightweight (need to register, sometimes pay) and powerful (do not support so many languages) like ideone. However, some of you may find it useful.
If you know about other online IDEs, let me know in comments!