Microchip Technology Inc.

Cost: Free
The MPLAB IDE is an Integrated Development Environment for the PIC microcontroller family. Out of the box, it comes with an assembler, but you can integrate your compiler into it. If also supports debugging with one of the PIC cable. In addition to the assembler, here are two components which are worth the price. It comes with a hardware simulator, and a stimulus generator. The hardware simulator allows you to step through your code without having any hadware. Very convenient when the prototype is not ready, or if you simply want to validate the operations of you code. The stimulus generator allows you to simulate activity on the I/O Pins. This is nice if you want to debug your I/O handling routines.

One of he quirk I noticed on the simulator is the steping over function call. When the system returns from the call, you will not break on the instruction after the call, but the one after. Only a minor annoyance.
If you are using one of the small processor, and do it all in assembler (like I am right now), MPLAB IDE is the de facto standard, and there is no reason to get anything else. If you are using one of the bigger PIC with tons (for a PIC) of RAM & ROM, and are programming it all in C, MPLAB IDE is still be a nice tool to have as you can use it as your single IDE (and not have to learn yet another one).
If you are developing something on a PIC processor, do your self a favor, and get the free MPLAB IDE.