MPLAB is a free, integrated toolset for the development of embedded applications on Microchip’s PIC and dsPIC microcontrollers. There are two different versions you can use.
MPLAB X
Experimental version, currently on development but includes many advanced features which ease the development process.
The MPLAB X IDE is the new graphical, integrated debugging toolset for all of Microchip’s more than 800 8-bit, 16-bit and 32-bit MCUs and digital signal controllers, and memory devices. It includes a feature-rich editor, source-level debugger, project manager, software simulator, and supports Microchip’s popular hardware tools, such as the MPLAB ICD 3 in-circuit debugger, PICkit™ 3, and MPLAB PM3 programmer. Based on the open-source NetBeans platform, MPLAB X runs on Windows® OS, MAC® OS and Linux, supports many third-party tools and is compatible with many NetBeans plug-ins.
MPLAB IDE
The classic IDE of Microchip which is on version 8. It is going to be stopped by the year 2013.
MPLAB Integrated Development Environment (IDE) is a free, integrated toolset for the development of embedded applications employing Microchip’s PIC® and dsPIC® Microcontrollers. MPLAB IDE runs as a 32-bit application on MS Windows®, is easy to use and includes a host of free software components for fast application development and super-charged debugging. MPLAB IDE also serves as a single, unified graphical user interface for additional Microchip and third-party software and hardware development tools. Moving between tools is a snap, and upgrading from the free software simulator to hardware debug and programming tools is done in a flash because MPLAB IDE has the same user interface for all tools.