The Arduino IDE isn't the most exciting tool in the Arduino chain - it's a big ugly and clunky, but it gets the job done. Alex from Dalpix.com has set about creating an alternative. Mariamole looks a lot like a 'regular' IDE such as Visual Studio and adds some much needed features for Arduino development.
Mariamole supports a 'Workspace' which allows you to work on multiple projects at a time. It lets you explore the source code of different folders right from in the IDE. This is something that it missing from the Arduino IDE, and makes working on larger projects (and projects with multiple boards) much more fluid. If you develop using Visual Studio, XCode or a similar IDE, you'll probably miss this feature from the Arduino software.
Mariamole also improves the serial monitor, allowing you to have multiple serial monitors running simultaneously. It also works as it should - you can keep it open while you upload a new sketch.
Mariamole available for Windows, Linux and OSX. It's all open-source and supported by donations. If you've had enough of the existing IDE, you can find Mariamole (and donate) over at the Dalpix website.