Flash2Advance Writer 2.0 Win95, 98, 98SE, ME [HERE]
Flash2Advance Writer 2.0 Win2K, NT, XP [HERE]
GBA FLinker [HERE]
LittleWriter [HERE]
EZ-Flash [HERE]

CoG [HERE]
Very nice all round image conversion tool

GBA Map Editor beta version 4 [HERE]
Created by Warder1. My map editor of choice and the one used in tutorial chapter 6.

pcx2sprite [HERE] (For Sprites)
Dovoto over at the Pern Project has produced this handy convertor for sprites

pcx2gba [HERE] (For Images)
Dovoto has also produced this handy convertor (use this one for converting image data e.g for 
Mode 4, 5 and 6)

bmp2spr [HERE] (For Sprites)
Created by Derek Payne, produces sprite header files from a Bitmap image, same output as 
pcx2sprite

Gameboy Advance Map Editor (for Linux) [HERE]
A map editor for Linux users, still in the early stages but already very useful

gfx2gba & gfxfront [HERE]
Converts many image formats to GBA image formats. gfxfront is a handy Windows frontend for 
gfx2gba

Objective [HERE] 
Sprite Convertor by TwoHeaded Software 

Framage [HERE]
Sprite Animation Tool by TwoHeaded Software

OpentUME [HERE] 
Versatile 2D Map Editor