If you are a programmer, or even if you just code as a hobby, you will know how important having a well equipped IDE is for the creation and debugging of code.
While web development can be handled easily using the built-in tools that come with popular browsers such as Google Chrome or Mozilla Firefox, certain other languages such as C++ or Java requires an IDE hundreds of megabytes in size. While it is strongly recommended that you go for a full IDE (Especially considering how most of them are free) if you are working on something serious, having to download and install a large file just for the execution of a few lines of code you picked up on the way can be a tedious process.
In such cases, online IDE's are the way to go. These are websites that allow you to write, compile, debug, and execute code while using minimal data as the IDE is located in the website's server.
I made a post about an online C++ shell a while ago, and now it's time to look at a Java IDE.
http://www.compilejava.net/ does the job beautifully well, no questions asked.
It claims to be a Java compiler created in PHP, and allows you to key in code or paste source and see the compiler as well as the program output as the program runs. You can even enter command line arguments if necessary, and the whole thing comes with a number of delightful markup formats.
Oh, the compiler is fully online, completely free to use, and no annoying registrations are required. Truly a great service!