Assembler - Simulator
If you want to develop PIC microntrolers based application, you need of course an assembler. The Linux assembler for PIC microcontrolers is a part of the gputils package.
Not sure about the program you wrote? Simulate it before programming! gpsim is a great simulator for PIC microcontrolers.
Integrated Development Environments
Perhaps you don't like the command line and you would prefer to click some button (You are a bad Linux user ;-) ). Don't be stressed out! Some developers thought about you.
PiKdev is the first IDE I knew. It support gpasm and C programming for PIC18 devices with cpik. PiKdev supports also some serial and parallel programmers. Unfortunately, development seems to be stalled.
Piklab provide more features. It supports gpasm, gpsim, the sdcc C compiler and some others. Piklab support various serial and parallel programmers but also Microchip hardware like Pickit, Picstart+ and ICD and some bootloaders.
KTechLab is a different IDE, designed for electronic and PIC microcontroller circuit design and simulation, using gpsim. KTechLab features an extensive circuit designer with autorouting and simulation of many common electronic components and logic elements. It features also an easy to use, flowchart based PIC program designer - Flowcoder; along with a BASIC like programming language called Microbe.
Electronic Design Automation
Kicad is the best open source (GPL) EDA software I know. Designed with wxWidgets, Kicad allow you to create electronic schematic diagrams and printed circuit board artwork. It provides also a GERBER viewer and a 3D viewer when used with Wings 3d
Others links coming soon.