The open-source Arduino Software (IDE) makes it easy to write code and upload it to the board. It runs on Windows, Mac OS X, and Linux. The environment is written in Java and based on Processing and other open-source software. This software can be used with any Arduino board.
Most intriguing features are:
Long-awaited new arduino-builder
This is a pure command-line tool which takes care of mangling the code, resolving library dependencies and setting up the compilation units. It can also be used as a standalone program in a continuous-integration environment.
Pluggable USB core
Your Arduino can finally act as a lot of different USB devices without any need to change the core, thanks to the new modular architecture. Libraries based on the new subsystem are already being developed!
Serial plotter
You can now plot your data in realtime, as easy as writing Serial.println(analogRead(A0)) inside your loop.
New goodies
For library developers, like unlocked examples while developing and optional linkage into an archive.
ArduinoISP example
Has been improved a lot and now you can flash your AVR chip using ANY other board.
Both Libraries and Boards managers
Notify if a library/core can be updated with a simple popup – no more outdated code floating around!
Note: Requires Arduino platform.
Also Available: Download Arduino for Mac