UECIDE: The Universal Embedded Computing IDE. For years the internals, or the "core" of the Arduino IDE has continued to improve. This is a good thing. However, so much effort has been spent making the Arduino core work well that the IDE itself has been somewhat neglected.
As well as that, many other boards have since been developed by numerous different manufacturers using vastly different microcontrollers. People have then taken the Arduino IDE and adjusted it to support different families of boards - MPIDE for the chipKIT boards, Energia for the TI Launchpad boards, etc. Again, this is a good thing; diversity encourages big improvements in the field, and one board that suits one job may be completely unsuitable for another.
This is why UECIDE has been born. We're not interested in the core of any one IDE, or any one family of boards. We are only concerned with the actual IDE itself and making it work right.
With so many different types of boards around now it is a bit silly having what is essentially the same program installed two, three, maybe four times. UECIDE aims to bring all the different board families under one roof, and to be able to program and switch between different boards as simple as selecting a menu entry.
Not only that, there are often different ways of programming the same boards. Take the Arduino, for example. Not only is there the standard Arduino core, but other people have designed their own cores, such as the popular Cosa core. A very different way of programming Arduino boards that is completely object oriented. We wanted to make using these different cores as simple as possible - again as simple as selecting a menu entry.
At the same time the Arduino IDE has a number of failings. Some parts, while they may have been perfectly usable at the dawn of the Arduino ecoculture, are now lacking in the kind of facilities that people demand. So we have ripped out pretty much all the code from the original IDE and re-written it. Almost the only thing left from the original IDE is the shape of the window.